3 months since you took delivery. The Consumer Rights Act 2015 only talks about length of time since purchase. I guess in this case the two, purchase/delivery are the same. You can look here for :
https://www.citizensadvice.org.uk/a...e-consumer-work/the-consumer-rights-act-2015/
If your cali is less than six months old the faulty item can be repaired or replaced and only if neither of these options are possible/available are you entitled to your money back (in most cases) - i.e. a replacement. If your dealer has offered to replace your faulty gearbox, as you say, that offer is within the rights laid out in the Consumer Rights Act 2015. Your only hope for getting your money back (replacement) is if the warranty from VW allows this or the dealer is feeling particularly benevolent - neither of which likely, but always worth a check!
Should you be happy with a replacement gearbox? I think so, Why? Well worse, in my humble view, could be they try and repair it, which they are entitled to do under the Consumer Rights Act, and who know's how long that would take or what other troubles could ensue as a result.
Advice? Accept the replacement, but insist on a loan vehicle during the time your cali is off the road.
Hope this helps - keep us all up to date on progress.