You can pay in sterling by cheque supported by a cheque guarantee card or sterling travellers cheques.
If you go to ov website - how to book - faq's - do i need cash onboard ? OR copy and paste this; http://www.oceanvillageholidays.co.uk/FAQs.aspx#7
this tells you all the ways you can pay onboard the ship. The trips are added to your onboard account.
Just advise them you want to pay by cash. You will have to leave a deposit with Guest Relations.