I will not suggest going to those PRIVATE money changers for large amounts, in Pakistan. Since you are selling property in Pakistan, best option for you would be to do a bank transfer. For your own safety & legal reasons. YOu simply can't bring money UK without showing a legit source of those funds; otherwise Inland revenue will be on your throat. I take since yuo are selling the property, someone (who OWNS the property) will go to pakistan? if yes then he/she can open a bank account there in no time and recieve payment in THAT account and then transfer funds to UK bank.
ADDENDUM: Open an internation banking personal account in Citibank UK. you can deposit money in that account from anywhere in the world.
This is the kind of option I was looking at. Yes, we are going back to pak in a couple of weeks purely to sort out the sale and so I guess our first step will be to open a bank account. Couple of questions though - do we open a foreign currency account or a rupee account? Secondly, since my husband and I both reside in the UK, how hard will it be to open such an account? Can we do it there and then by going into the bank with our passports and proof of UK address?