I would advise against this. Block block block her number.

I think because escort work in the US being illegal means more bad experiences with management and agencies. Either dodgy or exploitative management or management that does not engage in proper client screening; however, particularly in large cities (LA, New York), there are legitimate, upscale agencies. Do some research...I'm sure you're pretty good at spotting legitimate versus dodgy by this point.

Regarding working independently, the main advertising sites in the US are: Eros, RS2K, The Erotic Review and (the cheapest) Back Page. I believe the first is the most "upscale," the later two vary, and the least upscale is Back Page.

Client verification in the US typically involves either: pre-verification through an intermediary (RS2K offers this) or requests from a client for first and last name, work phone number and two provider references. Verification significantly reduces client volume; but it is  important because escorting is illegal (ie, the client could be a cop; or if not a cop, if the client is violent or wants to dodge payment, there really is no legal support.)

"Sugar Dating" in the US is huge and a "legal" way of selling and buying sex.  A lot of guys are looking for a girlfriend, but there are others who are looking for something transactional. The website "Sugardaddyforme" is particularly transactional...perhaps because it advertises on an escort advertising platform. The main negatives are having to pretend you aren't an escort, pretending like the client isn't buying sex, and less money. Positives, though, are that you don't have to pay for advertising and law enforcement is completely absent from these sites: in other words, you spend a bit more time ego-patting the client, less time making sure the client is not a cop/psycho. I think that "Brit girl looking for someone to have some fun with and help fund a holiday" would be gold: ie, be appealing and weed out people who want a girlfriend, people who want something long-term, people who want to give gifts or need-based support. 

I think also that prices vary significantly throughout the country. For example, the median hourly rate in Chicago is 350, in NYC is 500; rates are fairly low in Florida, I believe.

Anyways, hope that was helpful!

Positives, though, are that you don't have to pay for advertising and law enforcement is completely absent from these sites

I'm not sure I'd want to bet my liberty on that happy state continuing, especially as it seems screamingly obvious what's happening. Outside a Nevada brothel, it's always safer in the US to never say anything that a court could interpret as 'I will have sex with you for money'.

Paper mail forwarding services are ancient - lots of people and companies have a mail address something like 'Suite 1234, 9999 Regents Street, London' where that's really no more than one of hundreds of letterboxes in the wall of the real business at number 9999 who just stick incoming post in another envelope and post it on.

This is the same thing, except that it will do phone answering / forwarding too and has the prestige address of, erm, somewhere in Essex. (Apologies to anyone who really loves the county!)

If you have one, you get typically get charged per call / letter, so if they get lots of calls / letters, it could get quite expensive...