I would recommend the service the makers of Unison,
Panic,
provide. It makes it trivially easy to set up in this case as all the ports and addresses will be pre configured and hence it only needs a user name and password. It's Unlimited and quite cheap at $9 a month:
https://www.panic.com/unison/access.html
If you downloading files I would recommend a proper, paid, service as I believe the Virgin media ones have poor retention (period they keep files for) and download limits.