Dave
Below is a very simplistic guide I found online. If I were in your shoes, I would read the many online guides about how to improve/repair credit scores and have a plan of action. In regard to the defaults, if these are shown as being fully satisfied, you could write to the companies concerned and ask if they would remove the defaults now, as you have plans to move on in your life. They may just say tough or not even respond. If you wrote to the Chief Executives of the companies concerned in a heart warming way about your future plans, saying please many many times, you may strike lucky and catch them on a good day and they may arrange to remove the defaults.
I believe the defaults are the main factor for the mortgage being declined in February and obviously the credit score would not help. I have read other posts, where people with good scores have been declined. Getting mortgages at the moment is not that easy.
It may be worth you having a word with an independent mortgage broker, so that they can advise you of what may be available. Find broker using http://www.unbiased.co.uk/find-a-mortgage-adviser/ I think with 2 defaults and the current score, the options may be limited.
880 - 1000 You have a high credit rating - you have a good chance at receiving credit from the high street banks.
111 - 879
You could potentially have problems borrowing money depending on how much you are wanting to borrow and the type of product needed (eg mortgage, credit card, personal loan etc).
0 -110 You have a low credit score and will struggle to borrow money from a high street bank or building society and would have to speak to a specialist.
Huckster