If you're 17 (don't reveal on here as it's not allowed for under 18s) you can be referred to an adult GIC, but you won't be prescribed T until you're 18 at least. If you're 17 there's no point getting referred to Tavistock and Portman, or the child's GIC up north if that's closer (I can't remember the name if it).
On the NHS it isn't
hard to get T per se, but it is a fairly long process. All the GICs have their own processes, but as long as you're healthy in terms of weight, liver function, don't smoke and your mental health is being well managed then there shouldn't be anything stopping you once you turn 18. If you're not out you're not going to get T either though, you need to have lived as male for 6 months at least.
Give this a read through for how the NHS works:
http://issuu.com/bethseymour/docs/nhs_england_interim_gender_protocol/1I don't know so much about private, but they send to be around £250 a session and I've seen people be prescribed in two or three sessions, but I would imagine they'd be much more cautious if they were dealing with someone younger. When I was looking neither gendercare or transhealth would touch anyone underage, but in one of their cases they would if the parents were there too. I no longer lived at home at that point so it would have cost twice as much to get us all there as we're were coming from different parts of the country.
