GPS accuracy depends on how many satellites the phone can see. It's less accurate when there are tall buildings around because the satellites low on the horizon are blocked. When inside a building the phone uses wifi/internet which is even less accurate.
When you look at the location you should see a big circle around it which is the level of accuracy that the GPS thinks it has. If that is bigger than one building, you don't know which one he is in.
Right now my wife and daughter are both at home and on wifi, about 15m apart. But Find My Friends has them at least 50m apart - enough to be next door plus one.
The messages on this thread (from people who seem to know these things) establish that it's not viable to use a PI because the PI can negotiate a bigger fee from DH for hiding what you are paying him to find.
Although the amount does look large enough to be highly suspicious, it doesn't feel to me to meet a "proven beyond reasonable doubt" kind of test which is what you originally sought. There may be a rational explanation that you haven't yet asked for or been given. Sometimes you need to run up some large but legitimate expenses on business trips.
I also don't get that your DH "insisted he needed to go...to Bangkok". That's a very strange turn of phrase. If his employer needs him to go, then it needs him to go. But why does he have to "insist"? Anyway, you don't need to go to Bangkok for prostitutes, there are plenty in the UK if he is that way inclined.
It does just occur to me, is it possible that he is being blackmailed? Maybe you need to look for evidence closer to home, like a home PC, email etc., maybe webcam sites.
But overriding everything I get a sense that your trust level is so low that even if there is a genuinely innocent explanation you are going to struggle to accept it. So it seems that your relationship is shot either way. Sadly life will never be the same.