Right, seeing as you used pounds, I'm assuming you're coming from the UK.
Mobile contacts here are slightly different to what its like back in the UK when it comes to taking out a new mobile phone with the carrier. Back in UK, you get the phone cheaper or free, with you being locked into a tariff, paying more per month if you take out a more expensive phone.
Here in HK, the monthly payment do not subsidize the phone, rather you pay for the new phone at the beginning, and then over the course of the contract it slowly deducted off your monthly bills.
Over here you can simply take out a monthly plan without a phone, so you can carry on using your existing phone. Just make sure you have contacted your mobile carrier in UK first and have it unlocked (as its an iPhone) so you can use the sim card here.
My current plan is just a monthly one, without a phone as I had my own phone already. Just to give you an idea, I'm with One2free, paying $298 (thats around 20odd quid) per month with a 6Gb data limited.
Look into how much data you will be using (would you have wifi where you stay and work), and how much speed you need the data.