HSE do not, and never have had legislative powers, so therefore cannot relax any kind of legislation. They are an executive body who have powers to prosecute under guidance they set, but the laws and legislation granting them these powers remains the same and has changed very little in decades. But I get where the confusion comes from due to the nature of having an executive body with such powers.
Regardless, this still would have had an impact on the decision to close Vikings. Yes of course the poor reliability, ugliness and land mass of this ride would have been a major and balance tipping factor in the decision to close them, but the decision would still have been made balancing a number of factors.
Let's say for a minute that the HSE say "go nuts, do what you like", there's no way post Smiler and and post Splash Canyon that a company who plaster their parks with 'Danger of Death' signs, fence off innocent piles of rocks and don't let you wait on a platform for a train are going to do so.
So that means turning effects off, turning off wave machines and more doom warning signs, all of which make these rides less popular with guests. It means spotters, making these rides labour heavy and therefore costly to staff. These rides also have low patronage when weather is poor as often as it is in the UK and are extremely energy intensive to operate in a time when energy costs are high.
Although Vikings never worked and was ugly, it is more likely these factors sealed the deal in the decision making process on top of an already long list of reasons to bin it off. Rumba has most of these factors, it's just more reliable and is hidden more from view. Congo has most of these factors as well, also being more reliable, less ugly but takes up a significant amount of space.
Although I would personally love to see both rides with new, safer boats, soaking and interactive water effects, rethemes etc, we all know this is very unlikely. Commercially, these attractions will continue to eat into company coffers, take up room and offer poor guest experiences. The fact they're not as ugly and unreliable as Vikings only gives them short reprieves as far as I can see.