Look at it from the bike shops perspective . It will take roughly an hour . If the shop charges you £30 then that’s actually £25+ VAT . If the mechanic is on £10 per hour that leaves £15 for the shop but they have to pay the mechanic holiday pay , sick pay , employers national insurance contribution so they may be left with £10 . They still have to pay for the tools , the upkeep of the shop , rates , rent , power etc and , shock horror , they have to make a profit. They have to warranty any work that they do also . £30 to £40 doesn’t look too bad taking that into consideration , or comparing the hourly rate charged by other tradesmen .
If you really aren’t happy with the price quoted either try a different shop or do it yourself .