Are you eligible to work in the UK?
Access-Science Jobs is a job site where recruitment agencies and employers advertise current scientific vacancies. They may specify on their job advert that they are only able to accept applications from candidates eligible to work in the UK. This may be for a number of reasons, however generally it's because they are confident they can find a suitably qualified person with the right to work in the UK. If you are an international candidate, please appreciate the process in applying for visas can be timely and costly, and some companies may not be in a position to carry this out.
If you access the home office website you can access the list of companies registered to sponsor tier 2 visa's on the points based system. You can target your approach better in this way and I'd make it clear when applying that you require sponsorship.
There is a lot of procedures that the company has to go through and prove why they aren't recruiting someone local and why an international candidate.