I note you're in the UK.
The lack of AC causes a lot of the issues but, having been to the the carribean a few times with my ex who was over 200lbs at 5'0 I can suggest 2 things in the current heat wave: anti chafing cream and silk.
Fabric choice is a big deal, silk prevents rubbing and helps regulate surface moisture on the skin.
Further to these 2, there's the obvious keeping hydrated, balancing your salt intake to accomodate for increased sweating,
Again looking at native Pacific Islanders, carribean and middle eastern communities where female obesity is more normalised, there is a significant amount of acclimatisation that plays a role in easing the discomfort. Unfortunately the UK doesn't seem to be holding on to the heat past tonight (June 2nd) so sadly acclimatisation, which usually takes 15days for athletes and military personnel on tours, is not likely to happen.
4 years