For that budget and occasion consider something like a Montblanc Meisterstück Classique rollerball / ballpoint pen. Usually a 12-15% off voucher at goldsmiths too so come in around the £270—£300 mark.
It's a tough budget for a 40th, too high for the quality normal watches (Seiko, Orient etc), not high enough for the entry prestige brands.
If you deffo want to go with a watch then a tissot or hamilton sit at the price point and they are probably the best of the commonly appreciated and recognised swiss brands.
Depending on taste then this automatic tissot is a good price at £260 atm
https://www.goldsmiths.co.uk/Tissot-Automatics-III-Date-39mm-Mens-Watch-T0654071105100/p/17361021/
You will also find the automatic hamilton jazzmasters at the top end of your budget with some discount codes.
For the luxury brands like omega etc quality replicas run into your budget so if you see them at that price there are good chances of being fake and / or remote of being genuine but trashed with a service/refurb in the £400-£800+ mark.
If your brother appreciates Orient watches then there are a couple of orient stars in budget that are interesting like
https://www.creationwatches.com/pro...automatic-power-reserve-mens-watch-16888.html.