(EDIT:
c-w-s got there before me. again.)
The other potential issue is that the Y fare you didn't book would have probably been fairly flexible. The R fare could well be significantly less flexible.
no idea what exact fares you were looking at, but here are two fares with very similar prices, one R and one V:
LON-NCE/BA/REULGWUK
CANCELLATIONS
ANY TIME
TICKET IS NON-REFUNDABLE.
CHANGES
ANY TIME
CHANGES NOT PERMITTED.
LON-NCE/BA/VLV1Y
CANCELLATIONS
ANY TIME
TICKET IS NON-REFUNDABLE.
CHANGES
ANY TIME
CHARGE GBP 60.00.