Filtering is, alas, almost ubiquitous in the UK: you have to go to specialist ISPs to find ones that let you get unfiltered access, and even *they* egress-filter port 25 because if they didn't worm-infested Windows boxes would lead to their entire netblock being blacklisted.
My sister's using an ISP that explicitly egress-filters and bans everything other than HTTP and SSL in its almost-impossible-to-find terms of service (email? use webmail), and transparent-proxies the HTTP and HTTPS over some sort of carrier-grade NAT horror that drops all connections after half a minute or so on the assumption that a web browser will immediately re-initate it and no other application matters. She's happy with it, but when I go there there's no way I can ride an SSH connection over a TCP link that unreliable, not even with proxytunnel.
(I'll admit I'm wondering about some sort of interstitial thing that would make an intermittent TCP connection appear like a constant one, re-establishing where necessary, making this sort of thing work again. Unfortunately the client end would almost certainly have to work on Windows because most systems I encounter when travelling run Windows, and that is beyond my sphere of expertise.)