It's because the NHS puts up a variety of barriers to care, with most trans patients going through a 2-5 year wait to get the hormones. This applies to adults.
Most trans people in the UK use DIY until the NHS finally decides they're "Trans enough" to deserve it, and even then, it's not guaranteed to be available due to political reasons.
Essentially, the government in the UK is extremely transphobic, and trans people are demonized and looked down upon worse than immigrants.