Reneging on signed training contract in favour of a better one

I'd love some advice on the following matter, as I've struggled to find much information online and it appears to be a bit of a taboo subject.

I completed my law degree outside of the UK a year ago and did a number of vac scheme applications for firms in my home country at that time. I also submitted one to a London city firm, but really didn't spend much time researching city firms as I thought I was unlikely to get an offer. However, I was lucky enough to be offered a place on the vac scheme last summer, and, subsequently, on the training contract. I signed the contract as it was great to have the chance to train in London.

Since then, I have done a lot more research on city firms that particularly interest me, and have secured a place on a vac scheme at one such firm this summer. I much prefer the atmosphere at this firm and, because it has a much better reputation and offers a higher salary, training here would be a much better career move. If I were to receive an offer from this firm, is it possible to renege on the offer I received last year?