Cutting the chase, a credit score is a factual summary of your credibility. What it tells a lender is how you have behaved from a financial perspective over a period of years.
There are loads of good mortgage brokers in London, there are also a few not so good - it depends what kind of mortgage etc that you are looking for as to who would be a good idea to go for.
Try looking on a website such as unbiased they have reams of info on mortgage brokers in your area who should be able to help with your overseas needs.