Anyone can register .uk domain names on a first-come, first-served basis, but an address for service in the United Kingdom will be
required for registrants who live overseas. This address must be
listed as the admin contact for your domain name. This
requirement does not apply for .co.uk and .org.uk domain names. Source
So you only need a UK address for the admin contact for .UK domains. I have been working with domains for nearly 20 years and I have yet to have anyone attempt to contact me via the Admin, Billing or Tech contacts.
Given that forwarding services charge you on a usage basis, if you set up an address with one of them, it is unlikely to ever be used and if it is, then it must be really important and worth paying a few (insert currency unit here) for.