From pet detect FAQ:
Can all the scanners read the ISO chips used here in the UK?
All 'ISO' (Interntional Standards Organisation) scanners can read the FDX-B chip which is the one used in the UK. FDX-A chips were phased out some years ago but there are still many animals still alive with the 'A' chip, so it is very important to check with the retailer of the scanner, and ask if it will read both A and B. Most scanners do, but not all. The term used is 'backwards compatible'.
You need the scanner that can read both types. If i were buying i would spend that bit more to make sure any foreign chips are also read