Assuming you have less than �35000 why shouldn't it be safe ? All money up to �35,000 is guaranteed. Moreover B&B has been taken over by govt so money is probably safer with then than with any other bank. Finally, has any UK saver ever lost any money from UK bank failures. It simply won't happen.
That is assuming it's a cash ISA. If it's a stocks and shares one and the underlying investments bomb then you can lose - more likely some than all your money.
Justa minor amendment to my previous answer. B&B savings accounts have been taken over by Santander. The �35000 limit for B&B is still separate from the Santander limit of �35000. Money is still very safe indeed.