My learned detailing friends have given you good advice
As they have said, a polish is the thing that gives your car it's shine. A wax serves to lock in that shine and protect it from the worst of the weather.
So you do need to use both.
You say your local car wash has said a clay bar would be beneficial. Yes it would and an offer of £80 is cheap, which would raise some alarm bells with me. If they were proper detailers the price would be nearer £250. So personally I would avoid them.
Contaminants on paintwork dull the shine of a car, as do swirl marks and marring. Clay bars would get rid of this but as you sound like a novice (with the utmost respect) I wouldn't recommend that to you but you can use something called Iron X to remove iron contaminants and Autosmart Tardis to remove tar. These two will do almost the same job as a clay bar.
Iron X
http://www.carpro.uk.com/iron-x-500ml/
Tardis
http://www.polishedbliss.co.uk/acatalog ... -cat6.html
You spray on, agitate a little if necessary and then thoroughly rinse off. Don't let them try on the car. Can be used on your wheels too.
If you use Autoglym polish then I would suggest you top it with this:
Autoglym HD wax
http://www.amazon.co.uk/gp/aw/d/B001FEISS6
you apply wax very thinly, don't cake it on. You should barely be able to see it. Then buff off.
Always good quality, soft, thin pile microfibre cloths for buffing off polish and wax (separate cloths for both)
See this video for help on how to apply.
http://www.youtube.com/watch?v=pcNtTLci ... ata_player