Hello, my name is Ben and it is my pleasure to assist you with your question today. The general position is that an engagement ring is an absolute gift, meaning it was given to the other person unconditionally and without an expectation that it would be returned in the event that the relationship terminates. The specific section that deals with this is section 3(2) of the Law Reform (Miscellaneous Provisions) Act 1970 (http://www.legislation.gov.uk/ukpga/1970/33/pdfs/ukpga_19700033_en.pdf), which states that:
"The gift of an engagement ring shall be presumed to be an absolute gift; this presumption may be rebutted by proving that the ring was given on the condition, express or implied, that it should be returned if the marriage did not take place for any reason"This basically means that an engagement ring is given as an unconditional gift and the receiving party would have no legal obligation to return it if the relationship terminates and you do not get married, or the marriage fails. The only time it may be possible to overturn this presumption is if it was specifically made agreed between you that the ring is given on the condition that it would be returned in the event that you do not get married. You can't claim that there was an implied agreement just because you had agreed to marry - this must be specifically agreed between you so if no such agreement took place, then the legal presumption of this being an absolute gift would apply.
Hope this clarifies your position?