If the postcode check says that you currently can't get
any Freeview channels, then it's extremely unlikely that adding an aerial booster will help.
However, if the postcode check says that you can get a few channels now but you'll have to wait until the digital switchover for the full selection, you
might be able to get the full selection now. (I've had Freeview for several years, as have most of my neighbours, but the postcode check says that the signal isn't strong enough around here). However, you'll need a good quality aerial, such as this one
http://www.maplin.co.uk/Search.aspx?criteria=P T18U&DOY=29m2
with good quality cabling. You might also need a signal booster. (Adding a signal booster to an existing, 'bog standard' aerial is unlikely to help).
I suggest asking around your neighbours to see if anyone has managed to get a decent Freeview signal. If it's possible, someone will have almost certainly have already done it.
The alternative is to opt for Freesat (which is similar, but not identical to Freeview). If you've not got any satellite equipment, it will cost you �150. If you've got a dish but nothing else, you can buy a Sky box for about �20 on eBay and then buy a FreeSat card, from Sky, for �20. If you've got a dish and a box, you just need the FreeSat card for �20. Those prices are one-off payments. There's no subscription to pay:
http://www.freesatfromsky.co.uk/
Chris