There are no records for the UK, officially, from Hasbro in this time period. Although many of the ponies for 1984 would be sold here at that time, and would go on to be sold here in 1985, I think it sensible to assume that, since all the earliest UK variations or exclusives known appeared in the 1985 line, that this year was a year of imports where the UK was concerned. Very little survives from 1984's own UK pony release.
What we know about 1984 is sporadic, but, little by little, information is coming out of the woodwork. A UK card exists for the earth pony set sold in North America in 1983, and it is probable that this set was sold here in 1984/5. The ponies from it were certainly used to advertise ponywear in the 1985 insert.
An insert for this year does exist, and I have finally got my grubby paws on it. It contains the Earth Ponies set for 1983/4, the Show Stable with Lemon Drop, Pony Wear, The Grooming Parlour with Peachy and the Gymkhana with Butterscotch. It does not contain either the Waterfall or Dream Castle, and it claims to show the "whole" My Little Pony line for the UK, though no ponies are named. It is not dated, so we cannot be 100% sure if it is 1983 or 1984, but I suspect it covers the 1983/4 UK release. The logo from this insert appears at the top of this page.
The Argos Catalogue for Autumn/Winter 1984 shows a limited range of My Little Pony items: The Show Stable, Grooming Parlour, Bluebelle (although Argos's style was often to show one pony, and the text is hard to make out, it does not seem like this is a whole set being sold as it does not say "one supplied"), and the early plush ponies. There is only one page of My Little Pony in this catalogue, and so this may have been the official beginning of ponies being sold here (but I currently have no corroborative material to be certain).
It is worth noting that, although Baby Applejack is featured under 1984 with the rest of the Play and Care Baby Ponies set for the year, it's highly likely that, since Play and Care Set Baby Ponies first seem to have arrived in the UK in 1985, she was actually a 1985 release, and not a 1984 one.
Since the year on pony packaging is sometimes the year before the pony is released (due to copyrighting), it is really hard to know for sure, but this seems to make the most sense.