Good luck wherever you go,
The only thing I would say (and do a search on the threads for this) is do investigate what you get for your dosh.
Is the "all in" a guaranteed PPL pass or x hours of flying?
Beware "All In" means different things to different people.
Go to Southend and ask them how they can help you to get your PPL, what the costs are and what they actually mean, can they do you a deal if you pay in installments (but be careful with paying all upfront however - other schools have gone bust in the past), can they throw in your books etc as a part of the deal.
Try and speak with others who are near the end of their PPL at the school - I guarantee you they will be happy to tell you all their good and bad experiences.
Go and do the same at Stapleford
Hopefully someone will be able to point you at some other flying clubs accesible from London where you can do the same - sorry I can't help out there.
In answer to your question regarding "cheap" - make sure you do all your homework. If a deal looks to good to be true, it usually is. It could be cheap hourly rate but take 100 hours to get your PPL or expensive hourly rate but get it in 50. So the expensive then isn't.....
Personally I would rather learn from Southend (Same reasons as Freight) and then get a share later at Stapleford (but others will have a different opinion)