I think the dividing line between LCC and regular carrier is very thin these days. I mean, to a non frequent flyer what's the difference between bmi and easyJet? The seating policy is the only thing I can think about and some people (especially travelling alone) may prefer unallocated seats (so they can get a good seat by turning up to the gate early).

Now as a frequent flyer the differences between bmi and easyJet are more obvious. Lounge access, middle seat blocking and a free 'meal deal' are some of the benefits of diamond club.

At one time buy on board food was a sign of a LCC. Now many mainstream airlines do the same. Others have remained free but the offering has been pathetic. At least with buy on board then there should be some level of quality or few will be willing to pay.

Credit card surcharges were once the domain of LCCs, now many other airlines have copied (although none to the ridiculous levels of Ryanair).

Luggage charges have also been copied by many airlines, I can only think of US ones at the moment though.

Some self described LCCs allocate seats while others are open seating, so this isn't a way to tell LCCs and regular carriers apart.

Not all LCC's fly to far away airports. Although I can't think of any that use LHR (unless you class BD and EI as low cost) I think LGW is a good base for London.

Finally, it's not always the case that LCC's are cheaper. It depends when you book, etc. It's always sensible to check all options if you want the cheapest deal.