Here is an extract from the terms and conditions of one of the leading coach operators:
"You have no obligation to take out travel insurance for UK holidays, but we recommend that you have insurance against cancellation and curtailment of your holiday resulting from, for example, travel delay arising from adverse weather."
Unlike foreign holidays there are no uninsured risks that the operator takes which are likely to fall on him. Illness and injury (however caused) will be dealt with by the NHS. Should you be unfortunate enough to snuff it whilst away the tour operator is under no obligation to return your body home. The advice given by the operator I have quoted is sound but few people take insurance to specifically cover UK holidays.
Your operator seems to have drafted a contract (of which the conditions form part) which is unfair. I cannot think of a reason why a UK tour operator should insist on their customers taking travel insurance. There are no risks borne by them which insurance by the individual needs to cover.
If may be interesting to ask them why they have included such a condition in their terms of business. Also, since travel insurance varies widely in the cover provided, you might ask them and what are the risks that you are expected to cover to meet their requirements.