The legal way is to transfer money via Western Union, Money Gram like services. Western Union is quite expensive. They charge may be 1800-2000 rupees for transferring 20,000-30,000 rupees (sorry don't know about their latest charges but those would be few hundred rupees up or down) So I suppose if your relative sends money from UK to PK, the charges should be more or less same. But he should visit money exchangers there and ask the rates himself.
Once money arrive here, you can get it from any branch of that money transfer service. Every agent has got its own exchange rate. For example, suppose money is sent in dollars via Western Union, then one WU agent may issue you money for 105 rupees/dollar while some other WU agent may give you 103 rupees/dollar while the actual open market rate could be Rs 106.5 that day. (the difference between actual exchange rate and the rate they give you is their earning). The money may also be sent in local currency of other country.
