Well there may be various factors at play.
Firstly, the favourable exchange rate is likely leading to increased demand from foreign tourists, which in turn increases prices - cancelling out the rate benefit. Similarly, there may be higher demand from UK tourists favouring 'staycations' over foreign hols.
There may also be micro factors - e.g. major events that week in the areas you're looking at.
Have you considered Airbnb? There is an incredible amount of choice on there now, particularly in London.