If you sign in to My Virgin Media, you will find the Area Reference undeneath your account number (upper right hand side)
As far as I recall you can't set up a My Virgin Media account unless you know your Area Code ... so Catch 22.
Guessing here, but as Oxford is an ex-Comtel area, try code 31.
Cable customer since 1993. Services: FH TV, Sky Sports & Movies (2xV6), Talk Anytime Phone, VIVID 350, Virgin PAYG Mobile
As nodrogd correctly guessed, your area reference is 31.
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