As someone who is paid to advise on SEO (I'll admit it
) I would definitely advise leaving as is. Any possible benefit from registering .com for 10 years is negated by clouding the status of your inbound links (do you know how many you have? try the yahoo site explorer
Yahoo! Site Explorer or get a google webmasters account
https://www.google.com/webmasters/tools/).
You will 'probably' lose some google relevance for UK searches (there's another rumour - that .uk domains get a boost for UK searches).
Also, ignoring Google, a .uk domain makes UK folks feel like they are dealing with a UK company which can be a general benefit.