The first thing I would do is see how many harps are in the Harp dept at Guildhall. Then find out at what time does the dept /College close. The RCM now close at midnight to allow Harpists to practise longer.
If Guildhall close early, and you need a harp at your lodgings to practise I think Geraldine McMahon at Affairs of the Harp might be the best bet for renting, as Holywell don’t rent Concert Harps and Morley are very expensive. You should be able to get one via Affairs of the Harp for about £100-120 per month although most might not be new or in tip top condition. Bringing yours over from France would be an option, but unless you have two harps; one to keep at home on holidays, you would have to make the trip about 8 times a year. And that could be much more than hiring one here.
Good luck
Niina