Bear in mind that very few complete a PPL in the minimum hours so the all-in prices quoted are a minimum. Allow for at least another 10 hours.
Consider how often you would need to be going to the airfield for lessons and then tot up the cost of travelling by public transport; Southend, being local, could well be cheaper in the long run.
I suspect that most of the London Flying Clubs are not that easily accessible by public transport.
If you don't drive because you haven't learnt, I would recommend you do so. If you go on to professional licences, you will need to be mobile. If you can drive but, say, don't have a licence at the moment, then the logisitics will get easier when you get it back!
Cheers
Whirls