It is always worthwhile seeing if there is a tailor local to you who might be able to make you a suit. I would always want to know that I could have a fitting and adjustments with someone at some stage who I physically meet, personally.
The big advantage of finding a good tailor is that you can get the suit as you want it. Given the need to keep cool, think about asking about how you can reduce the lining, for example.
If you are looking for a RTW suit that you might want to get adjusted, normal recommendations apply as much to linen suits as to any others. I would recommend calling by at Slaters Menswear and seeing what they might have in. You might also try Austin Reed who do some linen and linen/cotton suits. It is all about trying things on and seeing what suits. Both stores do some adjustments as well.
I'd also point out that a lightweight (often called tropical weight) wool is well worthwhile bearing in mind. The general consensus is that they actually feel cooler than more obviously summer fabrics such as linen can do. Some such fabrics are very expensive, however.
Solid advice. Don't spend £750 on a bespoke linen suit whatever you do.