| Share-based compensation (Tables)
 | 12 Months Ended | 
| Dec. 31, 2022 | 
|---|
| Share-based compensation |  | 
| Summary of share-based compensation expense included in the consolidated statements of operations | The following table shows the total share-based compensation expense included in the Consolidated Statements of Operations (in thousands): 
|  |  |  |  |  |  |  |  |  |  
|  | Year ended  |  | Year ended  |  | Year ended |  
|  | December 31,  |  | December 31,  |  | December 31,  |  
|  | 2022 |      | 2021 |      | 2020 |  
| Research and development | $ |  6,264 |  | $ |  9,052 |  | $ |  4,417 |  
| General and administrative |   |  11,976 |  |   |  11,577 |  |   |  5,997 |  
|  | $ |  18,240 |  | $ |  20,629 |  | $ |  10,414 |   | 
| Summary of information about share options granted | 
|  |  |  |  |  |  |  |  |  |  
|  | Year ended  |  
|  | December 31,  |  
|  | 2022 |      | 2021 |  | 2020 |  
| Number of options over ordinary shares granted |  | 31,826,293 |  |  | 21,300,998 |  |  | 15,595,374 |  
| Weighted average fair value of ordinary shares options | $ | 0.37 |  | $ | 0.70 |  | $ | 0.59 |  
| Number of additional options with a nominal exercise price granted |  | 24,248,424 |  |  | 17,765,778 |  |  | 8,282,152 |  
| Weighted average fair value of options with a nominal exercise price  | $ | 0.51 |  | $ | 0.97 |  | $ | 0.85 |  | 
| Summary of all stock option activity | The following table summarizes all stock option activity for the year ended December 31, 2022: 
|  |  |  |  |  |  |  |  |  |  |  |  
|  |      |  |      |  |      |  |      |  |  |  
|  |  |  |  | Weighted |  | Average |  |  |  
|  |  |  |  | average |  | remaining |  | Aggregate |  
|  |  |  |  | exercise price |  | contractual |  | intrinsic value |  
|  |  | Options |  | per option |  | term (years) |  | (thousands) |  
| Outstanding at January 1, 2022 |   |  114,915,438 |  | £ |  0.52 |  |  |  |  |  |  
| Changes during the period: |  |  |  |  |  |  |  |  |  |  |  
| Granted |   |  56,074,717 |  | £ |  0.21 |  |  |  |  |  |  
| Exercised |  |  (5,823,534) |  | £ |  0.01 |  |  |  |  |  |  
| Expired |  |  (3,238,371) |  | £ |  0.69 |  |  |  |  |  |  
| Forfeited |   |  (9,389,161) |  | £ |  0.21 |  |  |  |  |  |  
| Outstanding at December 31, 2022 |   |  152,539,089 |  | £ |  0.44 |   |  7.0 |  | £ |  8,082 |  
| Exercisable at December 31, 2022 |   |  75,064,373 |  | £ |  0.64 |   |  5.3 |  | £ |  1,077 |   | 
| Summarizes information about stock options granted based on the market value at grant date which were outstanding | The following table summarizes information about stock options granted based on the market value at grant date which were outstanding as of December 31, 2022: 
|  |  |  |  |  |  |  |  |  |  |  |  
|  |  |  |  | Weighted |  | Average |  |  |  |  
|  |  |  |  | average |  | remaining |  | Aggregate |  
|  |  |  |  | exercise price |  | contractual |  | intrinsic value |  
|  |      | Options |      | per option |      | term (years) |      | (thousands) |  
| Outstanding at January 1, 2022 |   |  87,387,314 |  | £ |  0.68 |  |  |  |  |  |  
| Changes during the period: |  |  |  |  |  |  |  |  |  |  |  
| Granted |   |  31,826,293 |  | £ |  0.37 |  |  |  |  |  |  
| Exercised |  |  (318,222) |  | £ |  0.11 |  |  |  |  |  |  
| Expired |  |  (3,077,532) |  | £ |  0.72 |  |  |  |  |  |  
| Forfeited |   |  (3,340,390) |  | £ |  0.59 |  |  |  |  |  |  
| Outstanding at December 31, 2022 |   |  112,477,463 |  | £ |  0.60 |   |  6.5 |  | £ |  68 |  
| Exercisable at December 31, 2022 |   |  69,769,359 |  | £ |  0.68 |   |  5.2 |  | £ |  18 |   | 
| Summary of information about RSU-style options which are outstanding | The following table summarizes information about RSU-style options which were outstanding as of December 31, 2022: 
|  |  |  |  |  |  |  |  |  |  
|  |  |  |  |  | Average |  |  |  |  
|  |  |  |  |  | remaining |  | Aggregate |  
|  |  |  |  |  | contractual |  | intrinsic value |  
|  |      | Options |      |  | term (years) |      | (thousands) |  
| Outstanding at January 1, 2022 |   |  27,528,124 |  |  |  8.5 |  |  |  12,720 |  
| Changes during the period: |  |  |  |  |  |  |  |  |  
| Granted |   |  24,248,424 |  |  |  |  |  |  |  
| Exercised |  |  (5,505,312) |  |  |  |  |  |  |  
| Expired |  |  (160,839) |  |  |  |  |  |  |  
| Forfeited |   |  (6,048,771) |  |  |  |  |  |  |  
| Outstanding at December 31, 2022 |   |  40,061,626 |  |  |  8.4 |   | £ |  8,014 |  
| Exercisable at December 31, 2022 |   |  5,295,014 |  |  |  6.6 |   | £ |  1,059 |   | 
| Summary of information about stock options outstanding |  
|  |  |  |  |  |  |  |  |  |  |  |  |  |  |  
| Outstanding |  | Exercisable |  
|  |  |  |  |  | Weighted- |  |  |  |  |  |  |  |  |  
|  |  |  |  |  | average |  | Weighted- |  |  |  | Weighted- |  
|  |  |  | Total share |  | remaining |  | average |  | Total share |  | average |  
| Exercise price |      | options |      | contractual life |      | exercise price |      | options |      | exercise price |  
| £ | 0 |      |  40,061,626 |  |  8.4 |      | £ |  0.00 |  |  5,295,014 |      | £ |  0.00 |  
|  | 0.01 - 0.25 |  |  8,412,475 |  |  9.1 |  |  |  0.22 |  |  738,325 |  |  |  0.19 |  
|  | 0.26 - 0.50 |   |  38,339,879 |  |  6.9 |   |  |  0.41 |  |  14,287,927 |   |  |  0.42 |  
|  | 0.51 - 0.75 |   |  36,255,404 |  |  5.9 |   |  |  0.60 |  |  32,291,011 |   |  |  0.61 |  
|  | 0.76 - 1.00 |   |  24,103,880 |  |  6.1 |   |  |  0.84 |  |  17,446,630 |   |  |  0.87 |  
|  | 1.01 - 1.50 |   |  3,974,824 |  |  6.3 |   |  |  1.25 |  |  3,614,465 |   |  |  1.26 |  
|  | 1.51 - 2.00 |   |  1,391,001 |  |  4.7 |   |  |  1.70 |  |  1,391,001 |   |  |  1.70 |  
| Total |   |  152,539,089 |   |  7.0 |   | £ |  0.44 |   |  75,064,373 |   | £ |  0.64 |   | 
| Summary of the assumptions used to estimate the fair values of the share options granted using the Black-Scholes option-pricing model |  
|  |  |  |  |  |  |  |  
|  |  | Year ended  |  | Year ended  |  | Year ended  |  
|  |  | December 31,  |  | December 31,  |  | December 31,  |  
|  |      | 2022 |      | 2021 |      | 2020 |  
| Expected term |   | 5 years |   | 5 years |   | 5 years |  
| Expected volatility |  | 99-101% |  | 98-100% |  | 90-99% |  
| Risk free rate |   | 0.94-3.90% |   | 0.00-0.61% |   | 0.00-0.42% |  
| Expected dividend yield |  | 0% |  | 0% |  | 0% |   |