See, I'm on the fence with bill splitting because it's not always as straight forward as "it's only a couple of pounds"
It's ok splitting if everyone ends up paying the same but some may have had slightly less/slightly more food or drinks but in the case of the OP she has ended up paying more than anyone else but has actually consumed less - ie her meal plus one drink, the others have had their meal plus two drinks and coffee, this doesn't feel equitable at all so I don't think the OP is being unreasonable!
If I go out with my friends we all generally have similar things, ie one or two courses and one or two drinks so we just split the bill.
If we go out with my Dad there's me, DH, DS and my DF (sometimes his partner) and either my DF or I pay so fine.
If we go just with MIL and PIL we usually treat them because they are nowhere near as well off as us.
If we go with DHs whole family there are PIL (not so we'll off), SIL & her DH (similar income to us no DC) BIL & his DW (not as well off as us, one DD) and BIL (single dad with one DS) - clearly a different mix of wealth, some with DC in attendance some no DC and some who don't drink alcohol (usually only DH and I plus one BIL have alcohol) so its really not fair to expect some to subsidise others meals and drinks especially if they are not aswell off. In this case we just pass the bill round the table and each couple works out their own plus tip.