why would BT want over £5,500 and Vodafone had it fitted free?
When any provider raises an order it goes to the supplier (Openreach), as you never had FTTP before then Openreach will flag it needs a survey done first, when they surveyed your order, it was deicded ECC (excess construction charges) applied and BT raised that with you, BT don't just add costs because they want the order to go through to start charging for the service, you then decided not to go ahead and the order cancelled.
You then picked another provider, again a survey was done, this time no ECC was raised, a few options are possible:
The original survey by Openreach was wrong or right (we'll never know)
The 2nd survey by Openreach was wrong or right (we'll never know)
Vodafone accepted the order and swallowed the ECC, possible as each ISP wants to be #1 for FTTP
You'll never fine out the answer, BT added the ECC because Openreach flagged it, no other reason.