perfectly acceptable in the range of options available, some kind employers take the hit, some will do a sort of give and take on this, so you use some leave they give some time and pay you, the alternative is you take unpaid leave whilst you get back to work.
another alternative is you stay off sick until you are capable of getting back full time however in this case this would look very like toys out of pram to your employer (not me) or if you aren't people facing some working from home which you can factor travel time in order to "up" your hours, however this might not be the best advice from me not knowing your case and job.
on the plus side this will keep you sick time down as it doesn't count and if you are getting toward to the end of it (2 months is very generous on full pay), you will get back to work quicker than if you wait to be "100% fit" as 100% fit never happens, a good phased should be no longer than 4 to 6 weeks.
If it where me I would take the hit on pay as I only get 2 months full pay and as it is rolling if I got ill again in the next year I would be down to half straight away.
I speak from working in Occ Health and if you have, get a referral as what a GP states on the fit note does not always fit with what your job entails, I quite often have to recommend that a person actually stays off a bit longer due to the nature of there job in which case your fit note is used for your ssp
but always be aware you don't know what is round the corner, so keeping sickness down can actually help you keep a job if in 11 months you end up off sick again for a long period, as sickness is rolling.
sorry to sound so blunt, but your employer is not being unreasonable but sometimes employers don't think of all the other options, hopefully you can find a good option for both.