it would be either set in a configuration register, or set by adding/removing solder blobs on the circuit board. as for the ISM bands in the UK, i found [this document]
according to the document, the ISM band in question is 886-906 MHz, and the USA uses 902 to 928Mhz, which gives 4Mhz of overlap, so it's likely the reader works in that 4Mhz segment.