The troublemaker in me would have been tempted to put the £20 away, and tell her if she wants trouble, you'll call the police and we can see who gets there first.
In reality who knows what I would do in an unexpected and stressful situation like that. Quite possibly I would have folded and done exactly what OP did.
I do hope all comms were done with a punting phone? Presumably she doesn't know your name or address so there's very little she can actually do aside from be abusive over the phone.
A perfect example that even UKPers who on the surface of it think its ok to use their main mobile need to think again
PAYG never pay for the phone / top up by card
It would take MI5 type resources to link a mobile with a card & link the card to an individual
Much less resources to find a paid for mobile registered at an address
£250 tops & about an hour, maybe less dosh
Folding [as i have referenced above] is the best approach indeed
If i wanted to be cuntish i'd use an anonymous email & inform the local council that the property is being used as brothel
& provide the Awank link + address + mobile [& the friends mobile too]
If you have a printer also send em a letter unsigned without an address - from a 'concerned resident' stating the same
& include the profile + photos
That could cause whoever is renting or owns the property [& renting it out to SPs] some problems