Travel from London to Prague , the cheap way

You could take the Flixbus. There is a bus leaving London on Feb. 2. It takes 25:30 hrs and costs $71.98. You could fly, which will save you a ton of time and be more comfortable. The least expensive flight I see with a quick search is on EasyJet and is $91. It takes 1:55 min.

Easyjet. It is a bit short notice for the really cheap flights. Current prices for Friday 1st Feb:
Stansted to Prague £76.61
Southend: £65.50
Gatwick: £126.10 and £118.02 (two flights)

Website: https://www.easyjet.com/en/

let the train take the strain

https://www.seat61.com/Czech.htm

EasyJet.com

How much time are you allowing to go from London to Prague? If under a time constraint, fly. If not, then take the train, ie, option #2.

Option #2...If you take the EuroStar London to Paris Nord, stay a night in Paris, then take the early TGV train from Gare de l'Est the next morning to Frankfurt, just in time for lunch. Then take the train/bus to Prague.

I did something like your route in 2001, went from Paris to Warsaw by train, didn't think of flying then, still the Mrs would have been dead set against it, stayed overnight in Berlin after the 10 hr ride Paris Est to Berlin, and then the next morning another 6 plus hrs to Warsaw.

Normally I would say train as I love train travel, but that is quite a distance. Fly.

London-Prague is essentially same distance as Seattle -San Francisco.

Depends on your available time whether to fly or do surface transit.

Cost difference seems minimal. I would fly