well, both hk and uk has benefits.
eg, if you ( like a lot of hk people) constantly annually upgrade phones ( or even worse ever few months, it's also not uncommon for orientals to have more than 2 uk contracts), however, the cost of line rental is pretty high, eg, 25 quid for 200 mins cross network, however, the benefit is the latest handset annually at no/little cost, nokia 5800 mx for 50 quid or free (min even on subsided contract in hk is around Hk$1500)
but if you rarely upgrade your phone, eg, new phone on average ever 3 or more years, its far cheaper in hk as the line rental is very cheap and that you never worry about insufficent minutes, however, flipside is that phone cost (even of subsided with a long contract) is relatively pricey, eg, nokia 5800 mx even on subsided contract (at around HK$150 per month) in hk is around Hk$1500