I'm a second year mathematics undergraduate at a good university (top 50 world) in the UK who has started to take an interest into physics (mostly theoretical physics, i don't want to do any experimental physics). I've always had a interest in physics but never did perform so well on it (then again i never performed well in mathematics until later on but now I'm studying mathematics...), for those who live in the UK I only achieved a C in GCSE physics (Triple science) along with some other Bs (including mathematics), though at A levels I went on to achieve A*A*Aa (Maths, Chemistry Biology and Further mathematics AS respectively), so luckily i got into a good school despite my bad GCSE grades. When at university I started looking at theoretical physics and I'm now considering getting a PhD in it, or perhaps at least a masters (i was originally planning to do a PhD In mathematics). I'm not quite sure weather or not it'd be possible for me to pursue a PhD in theoretical physics due to my mathematics background... and the lack of physics background (though I'm only interested in the mathematical side of theoretical physics, string theory etc). To sum up, how would I go about pursuing a career in theoretical physics? Should i try taking certain modules or would i have to start a new degree (I wont be able to fund myself for a new degree actually). Just wondering on how i would go from a undergraduate mathematics background to a theoretical physics career? Thank you, and sorry if it doesn't make much sense, this was kind of rushed.