I think you have the advantage over anyone 21-22 without experience. I don't think a degree on its own is enough anymore. All graduates recruiters are after people with some kind of work experience, non-academic interests, and skills you have gained outside of your degree. After 5 years in the Royal Navay you will have a whole range of skills. You probably have developed a lot of self-confidence and initiative, so with a 2.2 or higher-you'll be a prime candidate
Graduating at 26 is not a problem at all. I personally have graduated at 26, and a top engineering company has employed me anyway. The same year I joined the company -last year- out of 23 graduates, I think that 4 of them were 25 and 3 where 26 years old. There might be still a tendence to recruit younger graduates, but if you have an added value, such as previous experience that is very valued by the companies.
I'm graduating next summer with an MEng at the grand old age of 25 and I've had no problems. If anything it's been an advantage as I've got more experience to talk about at interview and having been around a bit longer than fellow undergrads I've got more of an idea as to what I want to do when I do graduate.