Back to the OP.
Brexit has nothing to do with it, in fact it membership of the EU - for two reasons.
Firstly, a lot of the present problems are caused due to the Immigration Act of 1971, which was done at the behest of the then members of the EU prior to U.K. entry.
https://en.wikipedia.org/wiki/Immigration_Act_1971
Secondly, the pressure to reduce immigration in the last decade has been driven by the arrival of large numbers of EU citizens over which the Home Office has no control - so the resulting clamp down has been on those arriving from elsewhere.
As far as the status of forces children is concerned that has been an ongoing problem since the 1948 Immmingration Act, particularly those born in India. Spike Milligan, being an example. His parents were born in Ireland, then part of the UK, as it still was when he was born in India in 1918 when his father was in the army. When, after serving in the army in WWII he applied for a passport post 1948 he was told he was Indian and was refused. Thankfully the Irish offered him one immediately.