I guess doing a law degree would be a good start. You can't 'specialise' in aviation law at undergraduate level - I certainly haven't heard of any university in the UK education system that offers that*.
There are, however, a few universities offering aviation law as one of their postgraduate subjects (you need to have done an undergraduate law degree before you can do that*).
I think you could qualify as a solicitor through an alternative route such as ILEX but I'm not too up to date on the requirements. In any case if you want to be an aviation lawyer you need to undertake formal legal training.
*I'm talking specifically about law degrees, not talking about aviation degrees with law component.
P.S. I'm not a lawyer (far from it).