Re: Certificates of demonstrated ability?
The demonstrated ability thing applies to holders of foreign flight crew licenses. What some have done in your position is to gain say an FAA or Australian licence as these authorities allow Class 1 with any correction providing you can achieve I think it is 6/9 corrected vision.
The CAA then consider you to be an experienced person if you have I think it is over 500 hours. It is a lengthy process as it involves working in another country, usually as a Flight Instructor, but it is the only way to get a CAA Class 1 if you are outside the intial limit, which already has been stated as -5.
Good luck!