Using Pay As You Go abroad for a longer period

[GV1]

Hello, I have a O2 Pay Monthly account but looking to switch to O2 Pay As You Go and keep my old mobile number. Already ordered and got Pay As You Go SIM card. 
I am moving to an EU country but would like to keep UK phone number active for up to a year or so for online banking (approving payments) and to be able to receive texts or calls. 
What would be the cheapest way to do this?
Will I need to top up (buy Big Bundle) each month to keep my number active?

[Enlli]

To keep a number active you need to

A) Top Up every six months

B) Have a chargeable activity ( call, text or data) within that six months

[MI5]

@GV1 

You need to top up and make a chargeable call at least once every 6 months.

There's no need to top up just to receive.

No other restrictions on PAYG