JohnV85,
Firstly, congratulations on you decision to make the transition to aerobatics early in your PPL career - I've just got back from the Loop event at Leicester today, and it was evident from the grinning faces at the debrief quite how much an introduction to competition aerobatics has motivated the participating pilots.
I'd echo DJ's comments regarding the Extra 300 - you're right that its a fantastic bit of kit - I've been punting the one at White Waltham around for a couple of years on-and-off (including today) - but I'd say its overkill for Standard and Intermediate level - for which the weapon of choice is a Pitts, with a large nod to the Laser and similar 4 cylinder monoplanes. 6 cylinder, 300 HP aerobatics comes at 300HP prices - and even if cost wasn't a factor, I'd rather fly my Pitts S1 at Intermediate level, rather than the 300 - you're working 20% harder in the Extra, and for a while at least the additional performance will be working against you, instead of in your favour (in fact, one of the co-owners of my Pitts has just taken delivery of a shiny new Extra 300 - we're using it as a high-speed transport to the next competition, where he'll still be flying the Pitts).
Insurance for renting 300s is a variable factor - it can be the case that the operation offering rental have identified that the regular hirers have a certain number of hours, and negotiate a better deal from the insurers based on those hours - but its always possible to have named exceptions.
In terms of airfields around London, I'd say Waltham has the largest group of regular competition pilots, and has world-class instruction available. If I were you, I'd book a few sessions with Alan Cassidy in the Pitts S2 - you'll notice that a) the roll rate is a little more pedestrian than the Extra and b) the rudder response makes the Extra feel like a PA28 in comparison!
Feel free to PM if you're planning to fly at Waltham.
Cheers,
Ed.