I agree with Natalie......maybe bring a little of everything as it really can be quite unpredicatable.
Why don't you post again a week or so before your trip & we can can tell you what it has been like & maybe give you more of a better idea what to expect.
Just to clarify that's 28 degrees celcius or around 80 degrees Farenheit. But as said before the weather is very changable (which is probably why the British like to talk about it so much!!) and often quite wet at any time of year.
While it can get hot in the UK, it's very rare for us to have temperatures of 28C/80F. If we do, it lasts for about a day. That's why we visit Florida so often!