My approach to legal compliance is along the lines of ...
Some people will look for any opportunity to "make a quick buck" - and as a landlord, having anything not up to current standards gives the unscrupulous something to try and use to my cost and their gain. So for example I had RCDs on every circuit (actually RCBOs) before they effectively became required, and both our properties go beyond the minimum requirements for smoke and CO alarms. It doesn't stop people being a nuisance, but it certainly makes it a lot harder. Apart from that, I work on the basis that keeping the property up to date and in good condition helps get better, happier, tenants - and happy tenants tend to stay longer which reduces the costs of finding new ones.
So personally, whether legal or not, I'd be upgrading to something fully compliant with current building regs, just to cover my backside in case I come up against such a toerag. We did once have a toerag of a tenant. They scammed a significant amount off the neighbour (or rather, the neighbour's insurance) with what was obviously a fabricated claim - the damage claimed could not have been done by the neighbour without altering the laws of physics that make solid walls impervious to cars (think twin-leaf party wall separating the parking bays, and how one car can damage the other with that wall in between.) After the tenant had left, I found a letter while cleaning up showing that they'd tried to scam off the local council by claiming damage from a bin lorry at their previous address.