Here's an online supplier:
http://www.italianwinesdirect.com/epages/itali anwinesdirect.storefront
A web search suggests this as a possibility:
Monte's Italian Delicatessen,
23 Canonbury Lane,
Islington,
London,
N1 2AS
(That was from a page cached by Google. The shop's website is dead but the phone still rings so I assume that they're still in business. Call ahead to check: 020 7354 4335)
Another Google-cached page suggests that this wine shop, in Broadstairs, sells grappa but I can't find it listed on their current web pages:
http://www.thebottleneck.co.uk/index.htm
In Oxfordshire (Witney to be precise) you can buy grappa from the Oxford Wine Company:
http://www.oxfordwine.co.uk/index.html
Chris