Originally Posted by Globaliser

A Dual Inventory Fare. This requires inventory to be available in both of two separate booking classes, hence the name.

According to EF, for the OP's flight dates a T class seat can be booked using any of the following fares:
OLN8C2P4
QLN4C2P4
NLN7C2P4
SLN0C2P4
VLN0C2P4
LLN0C2P4
TLN0C2S4

To book one T class seat using OLN8C2P4, there must be at least one O class seat available and at least one T class seat available.

To book one T class seat using QLN4C2P4, there must be at least one Q class seat available and at least one T class seat available.

Etc etc. So if the flight has no O class seats, but has at least one Q class seat and at least one T class seat, then OLN8C2P4 cannot be used but QLN4C2P4 will be available.

Earlier today, I happened to have a look at availability on the OP's dates. On 20 September 2021, both of the MCO-LGW flights had gone to O0. So pricing with those numbers would have pushed the price up because QLN4C2P4 would have had to have been used in that direction instead of OLN8C2P4. But by the time I'd been through all of the experiment, O was back to O9. Very dynamic (or glitchy - but that makes no difference for what price comes out of the machine).