Hi B1dik and welcome. Your last line really answers your question. If you don't have the correct licence then irrespective of whether it's a straight swap or not, if you are caught or worse, have an accident then any insurance you have will be null and void and you'll be in a whole world of sh1t! If you don't have the licence, don't do it!
Also, the UK police will impound your bike on the spot and won't release it until you can show you have a valid insurance certificate to cover it. I have seen this first hand, the son of a workmate put a big bore kit on his Gilera scooter and got caught, it was impounded immediately and it took him ages to get it released and had to pay a massive storage fee!