It's worth mentioning that if you were to use your Avios to upgrade (and not use your 2for1) you can only go up 1 class. Therefore, to get into First you would need to pay for CW.

You can use a 2for1 on a one-way flight, albeit only the outbound from the UK. For our honeymoon 5 years ago I used Avios and 2for1 for First to Cape Town and purchased CW tickets home from Mauritius, which was not as expensive as I thought it would be for such a long flight (£800 each IIRC).