Russian and China will follow regardless
It makes no difference now if the UK limits itself to only its jurisdiction, the mere fact she's proposed seizing the worlds data from UK companies (and GCHQ has been caught doing it) will have two effects:
1) UK companies or companies with major UK bases won't be used abroad. It's a sovereignty issue, if you permit UK jurisdiction over your country, then you've surrendered the democracy to the UK. If you let your private data be grabbed by UK, then you have no private data regardless of your domestic laws.
2) Russia, China, every country on the planet will assert the same rights to UK data. She's provided legal cover for this. You can't even criticize them because Theresa's proposed the EXACT SAME thing. When they start grabbing bulk UK data from Google's and Yahoo and Microsoft and Vodafone, and Rackspace and every other company with a big UK base full of big UK secret data. What then?
We need 100% encryption NOW, URGENTLY.
We cannot protect MPs private data, government private data, or Parliamentary emails (WHY the f*** did you put Parliaments email into MS cloud?!!) from foreign surveillance by enemies and rivals and even nosey allies. We cannot protect trade secrets, business plans, journalistic exposes, none of it because of May and her ass covering, mass surveillance law.