A british certificate will need to be attested by

1. Solicitor in the UK
2. FCO office in the UK
3. Qatar Embassy in London
4. MOFA in Qatar

For the first 3 steps, you can contact Helpline group or Urogulf here in Qatar. They have these services as far as I'm aware and take a minimum of 25-30 working days. I needed to do mine urgently so I did it with an agent in the UK who charges around QR 1800 for the legalisation in just 2-3 days.

Hope that helps