You will need to rent a list. The only people with both sizeable and legitamate (opt-in or double opt-in) lists in the UK for postal and e-mail broadcasts are Experian and Claritas.
Expect a CPM (cost per thousand) of around £50 for postal and £100 for e-mail. These prices will rise if you need to add extra criteria to target your audience a little more than just living in a particular area.
Alternatively you could look at a CPA (cost per action/aquisition) deal where your client only pays based on results. Some companies such as IPT and PDV will take this on though their lists are predominantly e-mail based and are much smaller than than those of Claritas and Experian.
DON'T think you can get away with using a spammers CD as this will lead to trouble for yourself and your client.
The companies I mentioned above, as well as other smaller companies and list brokers, have been doing this for years putting in lots of time and money compiling their lists.
Perhaps your client already has a list of customers he wants you to mail? If not you should suggest that they start to build one as part of their CRM (customer relation management) system.