Do universities accept MPhys Physics graduates in theoretical PhD positions?
Yes they do. People change field all the time, and moving within a field when going from undergrad to PhD is perfectly acceptable, provided you can demonstrate that you have the interest and aptitude.
Do I have decent chances in being accept in a purely theoretical position?
Well, I know nothing about you except that you're a final year MPhys physics student, but that puts you in a better position than someone studying biology, or fine art.
Your chances will be improved if your dissertation is on a theoretical topic or if you have done any theoretical modules (eg QFT, GR, astroparticle physics, even cosmology). You should have good grades in these, and any mathematical modules you've taken.
The answers to this question: Will an off-topic Masters hurt my chances of a PhD place? may reassure you that you don't necessarily need expertise in the field you want to go into; having research experience in any field is generally sufficient, along with good grades. A good personal statement/ statement of research interests can also go a long way to convincing the admissions committee of your interest in the subject*.
I would also recommend speaking to your personal tutor/ dissertation supervisor/ other potential referees as soon as possible, as UK physics PhD application deadlines (for funded places anyway) are usually quite soon after Christmas.
*Please don't take this the wrong way, but it will probably help if you get your personal statement proofread by a native English speaker, or otherwise be sure to check your grammar very carefully (you made quite a few mistakes in your question).