The UK block isn't enough to stop them (come on. they're programmers - you have to assume they know what a damn proxy is.)
What is, is the fact that cheating sites will continue to exist in spite of everything. Perhaps the size of Sythe perpetuates more members, but in the end: there's a market for it. People who want to make and sell scripts, or trade on the blackmarket will find a place to do so (via google, at some hole in the wall if necessary), no matter how many sites Jagex shuts down. If they do try, it's their error: they're misdirecting their efforts.
The best way to shut this site down would be through more internal changes: better macro'ing detection, and associated IP bans. Closer "suspicious trade" inspection. etc. A re-work of the recovery system, to prevent recovs for ever being "unset", and instead merely changed. Scaring away members, instead of hunting the site itself.
Sythe has been around a long time. I'm sure if it were truly on the agenda, it would have been dones ages ago. It's just not worth the time for them to target sites, when doing so will merely scatter cheating - not diminish it.