If you are talking about which Network related to the Golf then it doesn't really matter as the car doesn't take advantage of the phones data unfortunately unlike other manufacturers.
The network to go with really depends on where you stay / work. 02 used to be fantastic where I worked as I could get a full 3G signal inside then a new building was erected and the signal is now terrible. In saying that its great everywhere else for me.
EE has 4G up and running in Glasgow and a work colleague showed me it on his phone. Wow it is fast. Its like using your home broadband on your phone. Lightening speed but expensive I believe.
I've just moved the whole family (5 of us) to GiffGaff that runs on the o2 network.
By doing this I've managed to take the monthly bill from over £100 down to £50 with everyone now getting a lot more data.
http://giffgaff.com/goodybagsIts a SIM only network at present although they are about to sell phones.
Going this route I found it cheaper buying my Galaxy S3 sim free and then getting the SIM. I was fed up being tied to a 24 months contract.
Now I'm free to change phones / sims whenever I like.