I shouldn't worry about insuring a 4 day trip in the UK. It would of course be deeply diappointing if for any reason you had to cancel the holiday, but if that were the case you might be able to exchange dates, or at worse you would lose the money.
Should you be taken ill on the holiday you would be looked after at the local hospital, just as you would be at home Unlike abroad when that service is chargeable along with the potential cost of being transfer to UK hospital, which runs into thousands and thousands of pounds.
Book your holiday and enjoy yourselves, just take care getting on and off that coach!