A few thoughts on this:
Providing some market sizing thinking for anyone revisiting this Q&A:
Remember that there's rarely a "best" answer with market sizing. What's important is that you break down the problem the way it makes sense to you. Importantly, break it down so that the assumptions you make are the ones you're most comfortable in.
For example, do you know all the major brands? Great go with that. Do you understand all the segments of that country's population (either age or wealth or job breakdown)? Go with that. Do you know the total market size of the tourism (or hotel) industry? Then break it down that way.
Hello! What is the best way to get from Turkey to the UK? Of course, I am going to fly by plane, but which is the best airline to choose? Do I need a visa there? By the way, where is it better to stop? renting an apartment or book a hotel? in any case, I will need a <a href=https://postcodefinder.net/england/london>London postcode map</a>, can anyone advise me a good one?