Said it before – will say it again:
Don’t get a ‘fashion’ suit. Get something classic. Therefore, most anything with a ‘designer’ tag should be avoided.
Buy as good as you can afford. M&S are well-respected for their tailoring and are a good OTP option if they fit your body shape. Lewin, Austin Reed and Slater are good places to look as well. Suits for £100 look like it and are pish and should be avoided.
Take SWMBO with you as you NEED someone else to check the fit. Don’t buy too tight (common problem) or too short.
Get decent shirts and ties as well. Shirts are cotton and ties are silk.
Shoes. Cheap shoes will drag a decent suit down. Leather, classic is best again – they will last longer.
Several people on here have recommended Reiss in the past. I went out of my way to check a Reiss store out the other week. Disposable fashion, not for suits.