Incall means you travel to her, at an address she keeps for work. Outcall means she travels to you, at your hotel or your home. That is the whole distinction, but it changes the price, the timing and the planning.
Incall
She provides the location, so she controls it. Incall addresses are usually flats in central areas with good transport, and you will normally get the street or the nearest station when you enquire and the exact address once the booking is confirmed.
It is cheaper, because there is no travel to pay for. It is also more flexible on length, since an hour genuinely means an hour rather than an hour plus two journeys. If you are in London anyway and do not mind a short tube ride, incall is usually the better value.
Outcall
She comes to you. For anyone staying in a hotel this is the obvious choice, and for visitors it is almost always the right one. You are not navigating an unfamiliar city at night to find an address you have never been to.
Expect to pay more, and expect a minimum duration. Ninety minutes or two hours is a common floor for outcall, because a one hour booking that involves an hour of travel each way is not worth her evening. Distance matters too, which is why every listing on this site leads with how far away she is.
What London hotels actually think
Very little, in practice. Central London hotels see guests receive visitors constantly and it is not a hotel’s business who visits a paying guest. A few points make it entirely unremarkable: give your room number directly by message rather than having her ask at the desk, meet her in the lobby if you prefer, and treat it as you would any other guest arriving.
Serviced apartments and some newer developments, Canary Wharf in particular, have concierge desks that sign visitors in. That is normal too. It is worth mentioning when you book so she is not surprised by it.
Which to choose
If you live here, incall usually wins on price and flexibility. If you are visiting, outcall to your hotel is simpler, safer and worth the difference.