I can’t stand Next stuff, I thought it looked cheap and didn’t personally fit me well. Got a suit from a wedding and it’s never been worn again.
Last couple of suits have been from House of Fraser, Alexandre of England, which makes me cringe a little (think it’s probably an in-house brand) but they’ve been really nice, have worn well, and for me the fit has been spot on.
Charles Tyrwhitt do perpetually have sales, I like their shirts, but never tried a suit. Slim Fit is actually the norm in shirts and designed for overweight City boys (and are like a tent on me with a 36″ chest/15″ collar), worth considering if the same holds true in suits and you’re not fat.