Firstly, I would recommend that you reconsider your current hosting provider. Interland / Dialtone had notoriously poor support, and is still the target of relevant and legitimate attacks due to its well-meaning, but largely incompetent network administrative staff.
Secondly, there are few disadvantages to having a server in the United States, and I would not say that it is necessary to buy servers from the UK for UK customers. In fact, US networks outperform UK networks by a margin of 3-to-1, and outperform European networks by a margin of 10-to-1 (Source: PC World, March 2004, "Hosting Analysis: Uptime".)
It depends on what kind of service you are offering for UK customers. If you are offering game servers would it because of ping and latency be best with server located in the UK, but if you are offering hosting and mail services for personal websites and small businesses would servers located in the US be a good solution, since you in most cases could save money because servers located in the US in general compared to UK servers are cheaper.