^^^ That's incredibly cheap, Mumstheword1!
This Bath-based tailor quotes £27 to £35 (or £50 if you want the sleeves shortened from the shoulders):
https://www.citytailors.co.uk/services/alterations-price-list/
The London firm says £22 to £25 for plain sleeves, £30 if there's a vent or buttons involved or £35 to shorten from the shoulder:
https://am-tailoring.co.uk/wp-content/uploads/2021/03/price-list-2021-November.pdf
Things are cheaper up in God's Own Country (Yorkshire) though, where the sewing services used by Rotherham, Doncaster and South Humberside NHS Foundation Trust are also available to the public. Their prices are 'from' £13 (or £16 for vents or cuffs):
https://www.sewnsew.org.uk/alterations-price-list.php
In the (probably very unlikely) chance that you're anywhere near to me in Suffolk though, MantaRay, I can thoroughly recommend this lady, who works from home and does excellent work at really low prices:
http://www.bee-deerepairs.co.uk/