Proportionately there's far more reports in this forum of mis-selling by field sales agents than other channels. You might get promised a better deal by a field agent, whether you'd actually get that put onto VM's billing system is a lot more questionable. Anything the agent says is very difficult to prove if VM dispute things after the event. I'd recommend you go with the normal online sales channels (ideally directly with VM). Regardless of field sales or online, if there's any vouchers or freebies involved read the small print carefully and keep good records - VM sometimes seem to struggle to honour those deals.
And finally, before signing a contract, make sure you know what to expect from VM - when it works and you don't need to speak to anyone then it's good. If there's anything that requires speaking to people (other than new sales) then the experience can be poor. Read through a few pages of Trustpilot reviews on Virgin Media - and personally I'd discount any review that has the word "Invited" next to the star rating - those are where the company has gone fishing for positive reviews, often after the most trivial customer engagement. All of this needn't put you off, all I'm saying is to enter any deal with your eye open.
