Im beginning to think that this so called compensation voucher of £100.00 is just a way of keeping the TA happy. The fact you need to use the same TA is a bit odd, and seems TA are putting up their prices when you mention the voucher. I was also told when trying to book with the voucher that it needs to be to the same destination.
Regarding the "incorrect fare" I was told by the TA it was a BA sale. Now they have my money for 21 WORKING days!