It's more symbolic than anything else. The Signal folks don't really care how effective this action would be. Sure, they can pull the app from the app stores for the UK region, disable any accounts registered with a UK phone number and/or block connection attempts from UK IP addresses. Some people will work around it, but many won't have the technical know-how to do so. The end result that's important, though, is that the Signal Foundation will be able to announce "due to anti-privacy laws enacted in the UK, Signal can no longer provide service in the UK".
I don't think it's as frivolous as you're making it out to be. By restricting the legitimate availability of the software it limits the use of it by entities more accountable than the average individual. Companies and governments themselves that use Signal will either be cut off from using the service or swallow the risk of using software they've circumvented controls in order to keep using. The likely result is that suddenly those companies and governments have a tangible motivation to pressure the UK (etc) to knock this shit off. It also sets a precedent for vendors of encryption-dependent software. Imagine if your bank followed suit and pulled out of the UK, arguing that eliminating the reliability and security of online banking would put their customers in danger? All of the above sounds pretty "effective" to me.
