Tenure-track offer with other applications in progress
I think what you are asking is if there is an ethical way to accept a position and then bail if you get a better position. Ethics are often very personal, but I think most people would conclude that this is unethical. That said, potentially the more important question, is can you get away with the behavior. From a legal standpoint, there is likely very little that can be done to punish you.

From a career standpoint accepting an offer and then turning it down tends to be a recipe for creating academic enemies. In your case, however, I think you can get away with it because of the Brexit and the fact you have a partner. If in the next month you get a better offer, you can tell the UK university that your partner has had a change of heart. Yes, I am advocating blaming it all on your partner to keep you from making enemies. Explain that your partner is concerned about moving to the UK because of Brexit and their treatment of foreigners. Yes, I am advocating telling a straight up lie. UK academics are generally up in arms about the Brexit. Having a change of heart because of the Brexit will be totally believable to them.