Welcome to our forums.
With regards to it crashing, have you ensured you have all the latest updates for TTN2 from their website? Does your PDA have the most recent ROM available? Are you running other programs in the background whilst TomTom is running?
There is nothing in the UK that uses the full postcode as standard but you can get a 3rd party application to run along side TomTom2 & 3 from
http://www.digitools.co.uk/ukpostcode.php which will allow you to search using a full postcode. I can't guarantee 100% accuracy though.