You do have different options but I would personally recommend a Tomtom package. The tomtom go 300 (I know its not a PDA but this would give you a very good sat nav package). The downside is that it may be a little over your budget at £368.95. Here is the link for that one:
http://www.totalgps.co.uk/section476.aspx
#If it is a PDA with sat nav you are after (personally this is what I use and would recommend) there are many different PDA's to choose from. If the main thing you are after is satellite navigation, I would recommend the following:
TomTom Navigation System with HP iPAQ 1710. This is the system that I am using at present and this gives me easy to use satelite navigation (tomtom 5) and will also allow you different PDA features. I use mine mainly for contacts and and for appointments in the diary. You can still have customised POI's (speed camera's, Mcdonalds etc) and unique voices for guiding you. The best thing about this system is that (a) Probably the cheapest Tomtom system on the market @ £292.58
(b) Works great as a basic PDA as well.
Here is the link for that system:
http://www.totalgps.co.uk/section478.aspx
I would recommend total PDA as I have used them before and have not had any issues with them.
Good luck