Yes, you're right CD, a way of slotting outsiders correctly into domestic competition.  Any player participating In an LTA sanctioned competition has to have an LTA rating.  Not sure what it costs now but used to be nominal or free (and still is) if you are a member of an accredited club.  I used to have a copy of a chart which gave a rough equivalence of LTA ratings to say French, US and other key tennis nations' systems which was used for this purpose.

A lot of Irish players have UK ratings.  Tennis is largely a summer sport there with almost no organised winter competitions and so keen juniors take a boat or a flight over to our shores for a weekend of competition from time to time  Over time this causes chaos in draws as their ratings rarely improve as they play insufficient matches here, but their standard often does.

Amy Bowtell, I think, trained for a while in the UK and Mariam Bolkvadze of Georgia currently does, being based at Chiswick and having spent enough time here to qualify to play for the Middlesex County Teams.