"The latest units advertised appear to cover 518-866 MHz"
Yes, that's the way they were advertised, but it was COMPLETELY MISLEADING.
UHF equipment does not cover the entire range, only parts of it -- and then only if you buy the right variant. For example (taking Sennheiser EW-300 Gp 1 as an example):
A covers 518-550 only
B covers 630-662 only
C covers 740-772 and
D covers 790-882 and
E covers 838-870.
If you've bought one for the UK, it will usually be the E variant, so will cover only 838-870. No amount of retuning will put it below 838. You would have to replace the internal board, assuming that they start up a production line to make boards for obsolete equipment, because there never was a range that covered 614-620.