Slater menswear Canon Street, they have a wide price range and do alterations for free
They are usually my first choice but just before Xmas they didn’t have much variety in suits.
If you fairly standard size marks will do you otherwise slaters is the only other place to go.
You can sometimes get a decent deal at Brook Tavener. Not sure how accurate their fecking tape measures are though.
[www.brooktaverner.co.uk]