I would like to know how I can register a .it URL in the name of a UK
Limited company from the UK.  Preferably via some sort of online
transaction using a credit card.

Hello ben_scammell, 

You can register .it domains with through a number of companies.  In
order to register a .it domain you must be located in an EU country,
but not necessarily in Italy. Here are a few for you domain
reigstration companies you could try (in order from most to least
expensive):

http://www.101domain.com/it.htm 
$180 for 2 years

http://websiteregister.net/it.htm
$180 for 2 years

http://we.register.it/
€ 37,80 for 1 year

http://www.tuonome.it/
€ 15,00 for 1 year

There are others as well, but these should get you on the right track.

Hope this helps you out,

Hibiscus

Excellent, just what I needed.

Ben

Another good site for <a href="http://www.livetodot.com">Italian
domain names</a> is Livetodot. They charge 30.00 GBP per year, and
claim to offer one of the fastest registration services of .it
domains.