The thing to remember with any no-frills airline like EasyJet is that you get exactly what you pay for.
They will charge you for any and every possible 'extra'. If you are checking luggage into the hold - they charge. If your hand luggage goes over their weight limit - they charge. If you want a glass of water - they charge.
They are often far from punctual about take off and arrival times, but the same can be said for BA.
They do not use the main London airports of Heathrow and Gatwick. They use Luton and Stanstead. Both LTN and STN have good public transport connections to the heart of London.
They do represent good savings for travellers, so long as you realise they do exactly what they say they will do and not one thing more.
Visit the EasyJet website, read everything carefully, plan how you intend to get to the airport, add up all the extra costs ( if any ) and if it's going to work for you, book your flights.