Give me a break, please!
Both Garibaldi and Mazzini were quite well known to the British, and I could not even envisage Palmerston signing off on such a plan to set up a hard-line egalitarian republic to turn into a protectorate (not even as an unlikely Plan C). Next thing coming up will be it was all a top-secret master plan set up by the Illuminati for their nefarious reasons
The British (and the French too - there were French as well as British warships in the straits of Messina when Garibaldi crossed over to Calabria) were for stability and for closing up in the most expeditious and less messy way the Italian crisis.
Additionally, the French had a vested interest in propping up Pius IX in Rome (but they were not truly willing to defend the Pope beyond the limits of Latium), and (even for someone this may appear to be counter to reason) the British too would have done everything in their power not to appear instrumental in the loss of the Pope's temporal power (certainly not after the Catholic Relief Act of 1829, or the troubles in Ireland).
But all of this is moot, after all: whatever the diplomatic players of Europe might have tried to set up, Garibaldi would not have sold out. He never did in his life, after all, and always refused to use his popularity for personal gains, either of influence or monetary.