Forget Business Premier.
Standard Premier - if you can find the £10 upgrade ones - includes a light meal and a drink, and better (wider) seats. For the sake of an extra tenner each (and getting brownie points from your girlfriend), I'd go for that if you can.
You haven't said when, so I've randomly thrown some guff together for you...
Once Eurostar is booked, you ought to get change out of 500 notes.
https://booking.eurostar.com/uk-en/train-search/standard/7015400/8727100?adult=2&outbound-date=2018-10-19&inbound-date=2018-10-21...