Never heard of a hotel that wasn't guest friendly in the red light areas of BKK or Pattaya. Most will say absolutely nothing when the girl walks in, either on her own or with you. There is no reason to be shy either, they will assume you are here for the girls. They will also assume that any Thai girl you are with is a hooker, Thailand has a strict class system and make no mistake we are dining out at the bottom of it.
Phuket can be different though. Some hotels there will want a fee for guests and I think the odd one doesn't allow them at all. I stayed at the Palmyra in Patong last year just after things opened up and I think the joiner fee was 1000 baht, which would actually be more than the room was at that time. These are hotels aimed at families or that have quite a few local travellers I suspect. A quick email to the hotel you are interested in booking will confirm everything. Some hotels also require the girls to be 20+ and not 18+. The hotels are not out to get you though with surprise bills, it just depends what guests they are aiming for, it is up to you to establish whether they are guest friendly which will not take you long. There is currently a publicised crackdown in Patts on hotels ensuring that girls visiting the premises are at least 20. How many will comply is unknown but they might do for a few weeks.
Wirral punts has some excellent advice. If I can add to that, I would say that flying to Bkk, taxi down to Patts (recommend Nams Lady Drivers for this) and then onto Phuket and flying home on a multi city ticket is a good way to see the country for the first time. It shouldn't cost you much more, if anything, to fly back to the UK via the middle eastern airlines and will save you spending a day travelling back to BKK. That is a pretty good starters tour to Thailand, it is very varied in places you will see but will also be punting heaven. Phuket is a great all rounder, not the cheapest and very touristy but there is so much to see and do and Bangla Road is still a great night out. From Phuket you can do day trips galore and it has pretty much everything you would expect a major tourist island to have. If you have longer than 2 weeks I would recommend Samui too. Plenty of girls around, a big massage scene there in Lamai and Chaweng and then pretty much every whore descends on Soi Green Mango at night, home to the legendary Green Mango club. A stunning place to visit and plenty of company to be had and the girls do not command gogo prices.
Thai Friendly, Tinder, Bolt taxis, Grab taxis (and food delivery) will take you far. Agoda is the Daddy for hotels in Thailand. Still worth checking Trivago to compare all sites but usually the cheapest Agoda rate is obtained by searching for the hotel name on google. This will prompt you to enter dates and you can get up to a third off at times compared to going to the website directly my experience.
High season is here and some things are getting silly, internal flight prices have rocketed, especially for the new year period. If you are going over Christmas or in January then a little bit of forward planning might be sensible. High season can catch you out at times. After January it is just Chinese New Year and Songkran to plan around but for most of the year you can travel to Thailand with almost no plans at all