While access to certain VCs may be an issue, being located outside the big city may actually be looked upon favorably by angel investors and VCs because this is a better allocation of resources -- you're investing more in your developers than in rent and other forms of overhead associated with being in London. Just make sure you can market your product sufficiently without having a physical presence in a big city. When approaching angels and VCs, be sure to present this setup as a strength rather than as a weakness.
It depends on what you are trying to do. Some things could be to:
We are currently based out of Cambridge, UK and have been finding it very conducive to web-based start-ups. Use the big cities that have lots of resources to find contacts, then keep in touch with them on a regular basis. A good question to ask yourself might be, is it necessary for me to be in a big city? Or can I just visit big cities, get the resources I want out of those visits, and continue to use those resources from a different geographic location? If its not necessary, you'll be just fine!
Being in a big city such as London allows you tap into a huge pool of talent. If you are going to be in a remote location, make sure that you have all the necessary talent already in your payroll. Gateway (the computer manufacturer) started in Iowa and soon realized that if it wanted to survive, it had to move to some place where talent was. Therefore they moved to California where lots of computer geeks work and live.
From my limited experience there seems to be a lot of startup support in Cambridge and it's probably more oriented around small companies than London would be.
The first thing I would do is try to make some local contacts that can point you to angels, lawyers, accountants and other resources you will need. It also does not hurt to be close to where you can recruit talent. While it's not critical that any of these people be close to where you start it's very helpful.
There's a bit of a Cambridge theme emerging in these answers. I'm someone who grits the startup oyster in Silicon Fen, and I have to say it's a place that has lots to offer startups - whether by basing here or by dipping into events.
In mobile apps, the communities you're connecting with tend not to be geographically concentrated. But still you may want to reach out to other developers in your neighbourhood. Why not start a pub-based meetup for mobile app developers? If you hit issues (for instance, with third party services whose APIs you're using or wanting to use), it's often a huge help if you can speak for a local developer group, rather than as just one more indie app team!