I have only bad memories of Viagra but thinking back I'm wondering if it was actually kamagra I had.
Its the same thing, Sildenafil citrate. Kamagra is just the Indian patent name for it, like Viagra is the USA and European patent name.
The only think you have to worry about is if you bought Kamagra in the UK. It will have been from an unlicenced shop and thus it could have been made with brick dust or god knows what else. So it could have been this other ingredients you had the bad reaction to.
My advice would be go to one of the large Asda's in the region (Gateshead or Benton) and speak to the pharmacist and get 4x100MG of Sildenafil generic. Cut them in half to get 50mg (or even in 4 to get 4x25mg) and see how you respond to this.
If you react badly then try going to your GP and asking for Spedra or Cialis - I say Cialis as a load of people on the forum swear by it, I never got any side effects from it, I just never got anything from it.