According to the official statement by the UK Government, you do not need to self-isolate when returning from Malaysia from 11/08/2020. There is still a law in place in Malaysia which requires visitors to self-isolate for 14 days upon the arrival. This, however, may be lifted soon as the travel corridor rules from Malaysia to the UK has just changed.
For this occasion, KLM has discounted some of its flights from London to Kuala Lumpur.
Flights are operated by KLM with 1 short layover (around 1.5hr) in Amsterdam. The whole journey then takes just under 15 hours.
The cheapest flight fare (Light) includes 1 hand baggage and 1 accessory bag and FREE flight ticket change (next, Standart fare with 23kg bag for an additional £30 one way).
The regular flight ticket price from the UK is about £480.