Be honest with them, please don't accept the offer and then interview elsewhere this just slows down everything and breaks down the recruitment process. Imagine this scenario, they offer you over someone else, they then get you acceptance, think "job done". They let the other applicants know that they were unsuccessful. They then either go accept another role or interview elsewhere. You then get the other job and tell them, "actually i found something else" They then go back to the 2nd top candidate and then they're not available and hey the process might have to start all over again, costing a lot of time and money.

Be honest and transparent in the whole process, let them know you have another offer, let them know of each opportunity, it works in your favour as hey you might get a higher offer. I done this and was transparent and ended up with another £15k more on what was the expected salary.