A quick google search confrimed that o2 (which is the GiffGaff network) launched LTE services on 2100Mhz in September this year..... to do so they must have retired 2100Mhz for UTMS (3g)
https://en.wikipedia.org/wiki/List_o...orks_in_Europe
Indeed it doesn't look like there is any UK operator still using 2100mhz for UTMS, so if you have the older chipset in your 5f you are foobar really.
Oh and I should also mention, that it is likely that 3g will be fully retired to make way for more 4g and 5g, but 2g is likely to stick around for longer.