Re: April 1st already?
Spot on with everything, apart from the very last line.
"More likely they would just block access from the UK, and let punters work a way round for their fix."
If I was a porn peddler I'd be looking at the cost of implementing age verification and comparing it with the likely future income from UK subscriptions. If the sums work, go ahead and implement age verification. If they don't, ignore the new laws and let UK ISPs pick up the tab for enforcement costs.
You're not going to get ad revenue from non-subscribers anyway - no-one is going to bother circumventing ISP blocks with VPN etc. and not also be running an ad-blocker.