Hi Tina, 10mm is an excellent lining measurement. I have been talking to ladies on an Ashermans board for over a year now, and the vast majority of us have very thin linings, its a "clue" that you have AS, not a fool proof diagnosis. The fact that you have a thick lining and the flow of your periods has not changed does make AS seem less likely. There are two doctors in London who are reknowned world wide as AS specialists, I went to see one (Dr Lower) for surgery last year. He charges £250 for a 4d ultrasound scan to look for scar tissue (and more for consultation). So the price of your clinic sounds similar BUT "ordinary" clinics and gynecologists would not recognise scar tissue if they saw it on a scan (Dr Lower has years of experience with this) but they can look for others things, like lining measurement. Make sure you go just before Ov and ask them to check your ovaries and confirm that they can see pre ovulatory follicles (these would be >11mm, and would confirm you are definately pre ov) thats when lining will be thickest and most "even" they should see a "triple stripe pattern" meaning that the lining is continuous and even, no bumps, missing bits or thin patches. After ov this pattern breaks down so you would have missed your chance to check for this if you go too late in your cycle. A bumpy, uneven lining pre ov, would be another indicator of scar tissue.
By "having your tubes checked" do you mean HSG? If so, I think its good price, but I am not sure, I had mine done over here in Costa Rica, medical stuff is usually about 1/4 of the price here than it is in the UK. (mine was £68) As well as checking your tubes are not blocked, HSG will also show up scar tissue, so if you get one, ask for a digital or hard copy of the films, so you can send them to an AS specialist - AS is really too rare for most gynecologists/fertility specialists to be able to identify. IMO the best AS specialist in the world (he has successfully treated thousands of women), is Dr Charles March, you can send him your HSG results ([email protected]) with a brief description of your history and he will reply with his opinion for free. He has replied to several of my emails and viewed my HSG slides for free, and regularly does the same for other women on the AS board.
Have you ever done charting? Its a good way to see for yourself if you are ovulating regularly or not, what length your LP is etc I thoroughly recommend it, together with tests it can tell you a lot. If you havent already read it, get a copy of Taking Charge of Your Fertility and read it asap. Also, (excuse the personal question but) how is your cervical fluid? 30 is not old (spring chicken to me, I am 36 
) but changes in CF as you get older can make getting pregnant harder, but there are things you can do to improve it (drink more water, EPO tablets, pink grapefruit juice and guaifenesin)
Hope you manage to get the bloods and maybe a scan too on the NHS, that would certainly help get your started. I think the HSG is worth paying for because even if it comes back clear, you wont have wasted your money because the "flush through" of your tubes gives you a little fertility boost. Many women (including me 
) get pregnant very quickly after an HSG.