Just book a room for two at one of the chains (I do this regardless so I get two sets of towels) and just check in. If they ask about the second person, and you are checking in by yourself, just say they are arriving later - normally this only occurs so they know if there will be one or two for breakfast. Never had an issue doing this with Travelodge or Premierinn. If using a premierinn don't check in online as they want a guest's name.
Then just arrange how to get her to your room - if she can walk in fine, if not meet her and walk in. Reception know you're expecting a guest/partner so as long as she isn't drawing attention (or is a regular at the hotel) you're unlikely to be stopped going in or out.