Xmas is high-high season, so hotels will be at their most expensive (but still reasonable in comparison to the UK). Flights will be expensive.
Whilst it means that there will be more punters around, the girls know this and it is rare for girls to 'go home' during this period.
It is the dry season, so it will be hot with less likelihood of rain.
You will still know it is Christmas, without it constantly being rammed down your throat.
People do go to Thailand for just 7 days, but by the time that you have travelled there, jet lag etc it might feel very rushed. I would say that you need at least 10 days.
Other than that, you will have the time of your life........
In Pattaya, Viagra comes to you! There will be sellers walking around bars offering it to you. Otherwise, dead easy to get from Pharmacies. Never tried to buy PrEP in Thailand. Maybe just visit a Sexual Health clinic UK before you leave.