Fortekor isn't available in the U.S. but an online search gave me this information:

"After oral administration, benazepril is rapidly absorbed from the gastrointestinal tract and hydrolysed to benazeprilat, a highly specific and potent inhibitor of angiotensin converting enzyme (ACE). Fortekor produces significant inhibition of plasma ACE activity for more than 24 hours after a single dose in cats and dogs.

Inhibition of ACE leads to reduced conversion of inactive angiotensin I into active angiotensin II and therefore reduction in the effects mediated by angiotensin II, including vasoconstriction of both arteries and veins, retention of sodium and water by the kidney and modeling changes (including pathological cardiac hypertrophy and degenerative renal changes).

In cats with chronic renal insufficiency Fortekor reduces the protein loss in urine and reduces systemic and intraglomerular blood pressure. Fortekor increases the appetite, quality of life and survival time of the cats, particularly in advanced disease."

Fortekor is flushed out of the system mainly through the GI tract, only a little through the kidneys, making it fine for use with CRF cats. However, I think Metacam needs to go through the kidneys and that would be why the vet won't want you to use it. I don't know if your vet will think Henry needs a potassium binder, it may not be something they use in the U.K.

Subq fluids is basically water with added electrolytes which is given just under the surface of the skin, and anyone can learn to do it at home. It flushes the kidneys and I've seen dramatic reductions in BUN and Creatinine levels in my cats after using subq fluid therapy even for only a few weeks. However, as with Epatikin, it may not be a treatment that they use in the U.K.