Comparing their figures with OSM data for hotels, guest-houses, hostels and B&Bs suggest that the map currently contains about 7,500 out of 32,000 of these establishments in England (i.e. 23%).
Roughly 15% are tagged as a hotel, roughly 6% as a guest house, 1% as a hostel, and under 1% as a B&B. There are also a small number of motels and some pubs with accommodation that are conventionally tagged. As usual, there is a smattering of weird spellings, and a few misunderstandings of tagging conventions, but these only account for a couple of dozen entries. They don't make a significant difference to the totals. More than 75% of establishments that provide serviced accommodation are missing from the data (or at least not easily found).
VisitEngland count all hotels together, and I've not been able to find a reliable-looking and detailed breakdown of different types of serviced accommodation. Smaller establishments seem to account for less than a third of the data, but I would think that they account for a lot more than a third of all establishments. Their trade association suggests there are about 25,000 guest houses and B&Bs in Britain. OSM contributors have located about 14% of that number (note - this figure relates to Britain, others to England). It's possible that some of these have been tagged as a hotel rather than a guest house or B&B, - but on the face of it, smaller establishments appear to be the most under-represented.
Looking at the distribution by local authority, it seems that either VisitEngland is particularly sloppy at counting hotels in the Midlands, or OSM contributors are particularly diligent at mapping them. For the rest of us, this might be a good time of year to fill some gaps before visitors and proprietors start gearing up for the 2015 season.
If anyone is planning to map serviced accommodation they will probably want to pick up tourism=hotel, guest_house, hostel, bed_and_breakfast or motel and amenity=pub with accomodation=yes.