it would be useful to know first the real reasons why you are applying to London, since the interviewer will probably understand if you use a pre-fabricated answer.
In general some possible acceptable answers include:
- Interest for sectors/clients strong in the UK
- Personal reasons (eg family relocation)
- Knowledge of the office thanks to personal connection and fit with its culture/focus
In general, every answer showing you are looking for more opportunities to grow compared to your home country is fine, I would just avoid mentioning exit opportunities, as could give the impression you want to stay just to get the brand in your CV and then leave.