If someone 18+ in the US wanted to have sexual conversations with someone who was 17 in the UK (over the legal age of consent) So long as no pictures are exchanged would it be ok to write a "story" - if you will- of the two engaging in sexual intercourse. Both parties are over the age of consent in their respective countries.
There don't appear to be any relevant sexual offences committed here. However, the 17 year old UK person may be guilty of the improper use of public electronic communications network contrary to Section 127 of the Communications Act 2003:
(1) A person is guilty of an offence if he—
(a) sends by means of a public electronic communications network a message or other matter that is grossly offensive or of an indecent, obscene or menacing character;
I cannot comment on US law.