you poor thing. i was taken seriously by my GP despite having at the time a normal BMI, so i would hope that you would experience the same given that you are underweight. the GP didn't weigh me in my case, but i don't know whether they would have done if i had been obviously underweight. they did ask me my height and weight (it was a struggle to be honest about that i have to admit)
well done for gettign yourself onto a more even keel, and well done for spotting when you're at risk again too
you know that you have to be really honest, however difficult it is, so that the gp can get you the help you need. i know that this is easier said that done. i would try to emphasise how bad it has been in the past, and that the intrusive thoughts and impulses (if you can describe them like that in your experience) are getting worse and you can feel yourself at risk of slipping down the same slope again. try to be honest about how bad it was at its worst, which i know is really difficult, but they have to know how at risk you are
it probably depends on how your MH/ED services are set up in your area, but i was referred to the MH services initially (telephone appointment), who referred me on to their ED specialist services (consultation in person). then i went onto a waiting list for a couple of months before a slot came up for treatment (cognitive analytic therapy). this is just my experience, you may need different treatment
your home situation sounds hard. is there anyone else who can give you support outside of HCPs?