I advise people to not get to fixated on the location of secondments at this stage.
The reality is a New York secondment is likely to be one of the most competitive in a trainee cohort, particularly where there is no need to have a second language, plus the prestige of the legal market and the glamour of the city.
Reality is, by the time you come to thinking about secondments you are needing to think beyond that. You probably won’t go to York if you want to be a competition lawyer, you probably won’t go to New York if you want to do Real Estate. You’ll be thinking much more about your qualification process (as most likely a secondment would be done in your final year) and trying to align your experience in your secondment seat to where you want to qualify.
For some trainees that will mean staying in London or going on a client secondment instead.
If secondments are important to you in where you are going to decide to apply to, then look at the range of secondments the firm has and think “would I be happy to go on most of these” because that’s really the attitude you need to take when applying for a TC. What I would strongly advise against is claiming you really want a NY secondment in a TC application, as this is super specific and even if you secure the TC is going to be unlikely given how competitive it is.