As has already been pointed out, we do not have Ultralight in the UK so presumably you mean Microlight.
Microlight licences use a different set of exams and whilst Microlight licences have been issued in the past on the basis of someone passing the UK PPL exams, there is no precedent for accepting your Austrian exams.
As the licence you obtain will not be a JAA licence, the exams you have taken will expire unless you obtain a JAA/EASA licence within the exam validity period.
If however you do your training on a European "Ultralight" with an EASA C of A which is an SEP then all should be OK however; you will need to do the UK radio licence as well.