If you are going to do it in the UK, you don't actually "convert" from fixed wing to rotary or vice versa. The two are entirely separate licences, not ratings on one licence as in the USA. It is true that most of the exams are common to the two, and you do get 5 hours off the minimum flying time if you do one when you already have the other. However, as most people take more than the minimum anyway this is not that important.
If you want to get both, learn in a fixed wing first. It is easier and cheaper and things like nav are the same. That way round you will be comfortable in the air when you come to do the rotary licence and it will make it easier to concentrate on the harder handling skills.
Good luck and hope you are well funded.