Hi OP, I'm sorry you're having to deal with chemo but you sound like you are in good spirits looking forward to Glastonbury which is great! Given your skin will be a bit more sensitive to sun damage because of chemo, you'd be best to have an arsenal of stuff.
I'm not sure from your post and title if you are looking for SPF makeup (as in make up with sun protection in it) or both sunscreen and make up. Sorry.
Anyway, I always find La Roche Posay are pretty good for sun protection - they do SPF 50 which I would recommend.
So you could apply a generous amount of cream or lotion sunscreen in the morning, and then get a top up spray like this to use during the day:
https://www.boots.com/garnier-ambre-solaire-spf-50-invisible-protect-mist-spray-200ml-10323716
I would always use a cream to start with for sun protection as a spray wont guarantee you cover every bit of exposed skin, but they sprays are good to have with you for a top up.
If you are looking for some sort of make up with SPF, I know some foundations tend to have a rating on them, but in reality you would need to use a ridiculous amount to get the stated SPF protection on the bottle, so it's better to use a standalone sunscream under your make up. Or you could try topping up your powder during the day with something like this:
https://colorescienceuk.com/products/sunforgettable-total-protection-brush-on-shield-spf-50?variant=7076584751169
I use this in summer to put on my face during the day, it means I can keep my sun protection topped up without messing up my make up. It is expensive, but lasts for a while as you don't need to use a huge amount.
Also you could try using some sort of lip protection with SPF, I know Nivea do cheap lip balms that are about SPF 15, but last summer I got a few of these in different flavours and they are nice, not too sticky and handy to carry in your bag:
https://bondisands.co.uk/products/spf-50-lip-balm-watermelon
You could also try wearing a wide brim or floppy hat to give your head a bit of extra protection, could look very chic!
Hope that helps.