I do not believe significant numbers of people will arbitrarily change sex to the point of causing this kind of problem.
What sort of numbers do you view as 'significant'?
The GRA was intended to be used for around 5000 people who suffered from gender dysphoria, and most were expected to have had GRS. We are now told by the trans lobby that the GRA isn't fit for purpose since only about 5000 people have one, and the medical requirements are too onerous.
The new figure for the number of transgender people is between 0.3 and 1%, so between about 200,000 and 600,000 people, many of whom have no significant medical interventions. If all of these people decide to get a GRC through self-ID, do you think this number of people is significant and big enough to cause a problem? Or do you think most still wouldn't bother, despite the fuss they've been making about the consultation and Liz Truss's statement?
If all of the trans identifying sex offenders who are currently in men's prisons (the number is estimated to be between 46 and 78) were put instead into women's prisons (which would be their right under self-ID), do you think this is a big enough number to cause a problem? Since such men have the most to gain from self-ID, it's likely that the uptake would be particularly high amongst sex offenders in prison.
Is your 'belief' based on any concrete evidence, or just a feeling you have?