This is Dr. David
I am sorry you have severe psoriasis.
systemic treatment with methotrexate may be the right next step for you.
there are other systemic treatments for psoriasis such as Humira (adalimumab) or Remicade (infiximab), or Enbrel (etanercept) but these tend to be more expensive and difficult to get in the UK
let me know if you have other questions.