I think I might be able to help on this one since I've done it many times, many different ways (and I'm not just talking about trips to England
)First question is whether you want to bring anything back or visit lots of places when you're there. If you do then you'll want to take a car, if not and you can get to an airport which a budget airline uses then it's probably more convenient to fly.
There are 2 options to cross the channel (unless you own a white Lotus which was used in a James Bond film). Ferry or Channel Tunnel. The differences between these 2 options are price and time. If you can take an early morning or late evening train then the Channel Tunnel can cost as little as £49 which is about €55 if booked well in advance. Ferries are much cheaper. I use Norfolkline from Dover to Dunkirk but you can choose Sealink or P&O Ferries depending on times/prices. I paid £27 1-way last month but there are all sorts of deals, just visit one of the websites.
The Ferry will take more than 2 hours by the time you've unboarded, the Chunnel takes 35 minutes plus around 10 minutes to unboard. The Chunnel is from Calais to Folkstone so if you're heading to London you'll have a 15 minute head start on the M20 than people arriving at Dover port.
You cannot take vehicles with LPG through the Chunnel (I think) and Ferries are sometimes at the mercy of the sea. If the swell is up then expect delays.
Hope this helps.