"How difficult is it in London - anyone had any experiences within London airspace?"
When you say "in London" it would be very hard to get much further over London than the route described previously skirting around the northeastern edge of the sprawl. The LCY class D zone extends in from the east quite a long way over the centre. In theory you could ask for a transit but, assuming you fly an SEP, the "glide clear" requirements (not to mention common sense) would preclude you from legally accepting such a clearance at a low enough altitude to remain within the D airspace. The LTMA (class A) steps down to ground level over most of central and west London so a no go unless you have an IR and a clearance.
Southend is a lot further east than north weald / Stapleford so you could follow a more easterly routing from Nottingham out towards Ipswich/Colchester bypassing London altogether.
Worth noting that all of airspace around the eastern edge of London is extremely busy due to the number of airfields and the fact that everyone is flying at 2,000-2,500 avoiding the TMA so a good lookout is a must!