I would think to practice aviation law you would firstly need to qualify as a solicitor and to practice here in England you would need to be admitted as a solicitor of the supreme court of England & Wales. Whilst there are foreign lawyers working in London, their roles are very limited as jurisdiction is very important in the practice of law. Therefore you may be qualified to practice in England but that won't allow you to practice in France, unless you work for an English firm with an office there but even that isn't strictly speaking pracitising French law.
May I ask why you want to become a lawyer specialising in aviation? Remember, at the end of the day its 90% law and only 10% aviation-related, hence a fundamental knowledge of the law is a pre-requisite.