Found some cheap flights on skyscanner to Bali from London Heathrow in May. Looks like for the best price fly on Tuesday.
Fly on Garuda airways which I believe is the national airline for Indonesia. There is a short onward stopover at Jakarta and a return one at Singapore. You could include a city break in Jakarta by paying around £35 extra.
This has now been reduced to £390.47. See below.
I think this works over half term as well.