BullionVault ( https://www.bullionvault.com/ ) is widely used. They have a reliable reputation and they also have a base in London. The only downside is their association with the Rothschild family, but as for the site itself it has very good reviews.
If you plan on buying gold with a view to possibly selling it in future, be careful to do your research on UK taxation laws, especially 'Capital Gains Tax'. Here are a few links that you might find helpful:
A very good introductory guide to investing in gold:
Important information on Capital Gains Tax:
List of gold brokers:
I would advise that you take your time to do your research -- learn as much about gold investment as you possibly can, weigh up your options and proceed with caution. Try to work out what's best for your personal circumstances, discuss it with friends and family, and try to get professional investment advice if possible. As a general rule, never invest in something unless you understand how it works and are confident that you can make informed decisions.
Oh, and whatever you do, stay away from 'Rosland Capital'. They are scam artists of the worst kind. There are lots of Gold investment companies that advertise themselves on TV and online, but you're much better off going with larger, widely used organizations like BullionVault -- look around though, and do plenty of research before you commit to any investment decision.