MH conditions in the UK are woefully underfunded, as is funding for a lot of physical conditions - hence the immense waiting lists.
As others have said, the funding for weight loss drugs is not being taken from elsewhere. Health funding is incredibly complex, but in its simplest form, they've identified weight loss injections as an invest to save.
If co-morbidities currently cost the NHS £2 billion a year, but investing £10 million in weight loss injections a year means that in five years time that will be halved to £1 billion a year, then it is a very good thing (and will make more money available for other conditions like ED in the long term). - (Caveat - numbers completely made up just as an example)
The problem is it is very difficult to make the same invest to save calculations for MH conditions - there needs to be a lot more research and understanding of how they can be identified earlier and what interventions can be put in place early to save the money down the line. Unfortunately the triggers and early warning signs are not well understood to be able to do that effectively for any mental health condition.
I'd like to see more of an investment in school and community counsellors - there is not much I admire about the American school system, but I do like the fact that many secondary schools have counsellors on staff that students can access when they need to for early support and a lot more awareness being raised among young people about things like Kooth (think along the lines of Esther Ranson and Childline adverts from the 80's and 90's).
We need to get some of the key social media influencers onto it (because as much as I hate it, it really is a good way to get through to kids) - get people like KSI, the Sidemen, Mr.Beast and KMK involved in campaigns and you would reach a lot more young people who would then be able to get help earlier.
If we did that, along with looking at other things that are equivalent to weight loss injections where investment now will save a lot more later it would take a lot less time to get the NHS on track and make sure the money is there to treat more severe conditions.
Unfortunately, the system has been so decimated from years of underfunding and unnecessary change that it lacks the capacity to be creative.
The government could also fund it quite easily. However there is such a lack of understanding about how national debt works, with most people believing it works the same as household budgets, that if the government borrowed the necessary funds to do this, Reform and all of the other anti-public sector folks would have a field day!
Edited to caveat about costing being made up as an example