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Earnings per share (Details)
$ / shares in Units, $ in Millions
 12 Months Ended
Dec. 31, 2023
USD ($)
$ / shares
shares
Dec. 31, 2022
USD ($)
$ / shares
shares
Dec. 31, 2021
USD ($)
$ / shares
shares
Dec. 31, 2023
£ / shares
Dec. 31, 2022
£ / shares
Basic (loss) / earnings:          
Net (loss) / income from continuing operations | $ $ (2.6) $ 32.0 $ 30.0    
Net income / (loss) from discontinued operations (including net gain on disposition of $6.6m in 2021) | $ 0.7 (5.1) (0.1)    
Net (loss) / income | $ $ (1.9) $ 26.9 $ 29.9    
Weighted average number of £0.50 ordinary shares:          
For basic loss / earnings per share (in shares) | shares 26,897,556 27,304,847 27,698,691    
Dilutive effect of potential common stock (in shares) | shares 123,403 236,355 333,815    
For diluted (loss )/ earnings per share (in shares) | shares 27,020,959 27,541,202 28,032,506    
(Loss)/earnings per share using weighted average number of ordinary shares outstanding:          
Basic earnings / (loss) from continuing operations (in usd per share) [1] $ (0.10) $ 1.17 $ 1.08    
Basic earnings / (loss) per ordinary share for discontinued operations (in usd per share) [1] 0.03 (0.19) 0    
Basic (loss) / earnings per ordinary share (in usd per share) [1] (0.07) 0.99 1.08    
Diluted earnings / (loss) per ordinary share for continuing operations (in usd per share) [1] (0.10) 1.16 1.07    
Diluted earnings per ordinary share for discontinued operations (in usd per share) [1] 0.03 (0.19) 0    
Diluted (loss) / earnings per ordinary share (in usd per share) [1] $ (0.07) $ 0.98 $ 1.07    
Net gain on disposition of discontinued operations | $ $ 0.0 $ 0.0 $ 6.6    
Ordinary shares          
Class of Stock [Line Items]          
Common stock, par value (in GBP per share) | (per share) $ 0.50 $ 0.50 $ 0.50 £ 0.50 £ 0.50
[1] The calculation of earnings per share is performed separately for continuing and discontinued operations. As a result, the sum of the two in any particular year may not equal the earnings-per-share amount in total.