Assuming this isn't one of 'my' old Twilight Zones, the £1 coin input is D1 (left), then 50 pence D2 (centre), 20 pence D3 (right) and 10 pence D4 (4th), so the values for 'flat' £1/play would be in decreasing order, 10, 5, 2, 1, with 'units/credit' at 10, 'units/bonus' and 'bonus credits' at 0. You could use these last two for an incentive to insert £2 for 3 credits by setting them as 20 and 1 respectively.
Re. the 50 pence; I can't think of any reason why, but it may be locked out or 'inhibited' by the coin interface board in the cabinet. AIR, the lowest 4 of the 8 DIP switches on the interface board are used to enable/inhibit the first four 'channels' of the coin acceptor. So they should all be set the same.