When you are thinking about changing the rules, you have to consider two aspects; human nature and the Covid virus itself.
If the quarantine rules are relaxed for the two states, you have to think like a chess player; you have to think at least one move ahead.
If the government relax the quarantine regulations, then what will happen? Those from mainland Spain will use those two states as a travel conduit to get away from Spain. A majority of those people will come to the UK.
Then, not only will the NHS be flooded, so will the rest of the country and those coming from those islands will just infect British people and then we will have a second wave