Under assured shorthold tenancy there is right to carry out repairs, but no specific rules about entry. However, a court would likely uphold that to carry out repairs it is implicit that entry would be required and thus allowed.

Under other std tenancies, they have the right to enter after 24 hours notice to effect repairs that are necessary and inspect the fabric of the building.

Regardless of this, there are rights and common decency though. I would consider telling her you are deducting the cost of the bed, a new hamster and a cleaner from next months rent. However, bear in mind that she could (if she's a idiot and a tw*t) evict you with 2 weeks notice if you are out of contract.

Chris