I have similar memories, with pre-start exposed heat in 2011 and wide temperature ranges then and 2015.
The forecast will bounce around a bit but, unless it changes markedly, it looks like it will be far hotter than either of those, when I think I remember hearing the maximum was around 28. And, as they say, it's not the average temperature that gets you, it's the extremes, so cooler weather towards the coast won't make the inland bit any easier. I've not done a hot UK audax since the Buckingham Blinder at Easter 2011, but I recall quite a few reports of people struggling this year in temperatures around 28-30.
I don't mind the heat too much. I've ridden in 40+ and full sun in the Balkans and Australia and have learned how to cope with it, but I find it pretty unpleasant when it's above body temperature. My main concern is my stoker, who I think will struggle to drink enough to avoid dehydration if it gets as hot as it appears. And if he does have problems, I expect he won't be the only one!
With any luck it might settle down and not get above the early 30s, in which case it will be very pleasant! But I've added rehydration salts to my kit list!