Stayed in many good hotels in West London and never had a problem except on one occassion she was stopped (this hotel was only one I noticed a porter guy on the entrance - basically opening door for you) and she told them she was visiting a friend - they called me and asked if I could come down and pick 'my friend' up. Just give her your surname and room no and even if stopped she should have no problem. This girl never had my name that is why they asked me to come down.
Some hotel with lift card she has even come in lift behind guests or you could go down and meet her. Any large hotel they will easily just be able to walk in. Even if small hotel will allow guests. Only thing I can imagine is some small hotels ask how many people room is booked for so just book room for 2 people at booking.