I have had this issue with BT, TBH I would just phone them up to make the order, their automated system might be broken, it does happen.
I did some checks and I can see its available for door numbers 1 to 14 of that postcode, I have checked on the BT Wholesale Checker along with the BTOR Where and when Page, both say its available.
[Exchange Information]
Exchange: Exeter
Status: Accepting orders
[Rollout Information]
PostCode: EX42PN
Address: 1, Snowdrop Mews, Exeter, Devon, EX4 2PN
PCP Cabinet: 135
PhonePole: dp6701
You have the following Fibre Products available to you:
FTTC 80Mbps (down), 20Mbps (up)
Fibre Cabinet: 135
FTTP 330Mbps (down), (not defined)Mbps (up)
...
[Rollout Information]
PostCode: EX42PN
Address: 11, Snowdrop Mews, Exeter, Devon, EX4 2PN
PCP Cabinet: 135
PhonePole: dp6703
You have the following Fibre Products available to you:
FTTC 80Mbps (down), 20Mbps (up)
Fibre Cabinet: 135
FTTP 330Mbps (down), (not defined)Mbps (up)
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[Rollout Information]
PostCode: EX42PN
Address: 14, Snowdrop Mews, Exeter, Devon, EX4 2PN
PCP Cabinet: 135
PhonePole: dp6702
You have the following Fibre Products available to you:
FTTC 80Mbps (down), 20Mbps (up)
Fibre Cabinet: 135
FTTP 330Mbps (down), (not defined)Mbps (up)
So like I said I would just phone BT up and try to order over the phone, that way if any issues pop up they might be able to work around it.
Paul