OK, presuming you booked before the fares increase at the start of the month? If so, it looks like you bought 2 of
these £19.25 fares, which have ticket code OCU.
LNER has 25 levels of prices for both their standard and first class Advance tickets between York and Newcastle, and the OCU fare is the 15th most expensive first class Advance. Therefore for a comparable standard fare, I'd look at the 15th most expensive standard Advance. This is
ticket code BTP, which costs £12.90.
The difference here is £6.35. £19.25 is coincidentally also 50% more than £12.90, which is a pretty typical uplift for first vs standard.
If we want to look at walk-up fares, the cheapest valid first class fare would have been the
£24.90 Off-Peak First Single (route LNER only), whilst the cheapest valid standard class fare would have been the £18.15 Off-Peak Single (route Any Permitted). So the difference would have been £6.75.
So I think we can take the difference to be approximately £6.50. Based on my above suggestion of perhaps expecting half back, you'd be looking at £3.25 each or £6.50 in total. Of course your cost of buying a 'replacement' meal would likely be more than this, so I'd revert to the minimum of £10 each.
If they offered a £20 Rail Travel Voucher I might be minded to accept that.
I would suggest an email is less likely to get lost