We spread out the 1 year vaccines for our son after he had some strong reactions to his baby ones (and that was after refusing Rotarix and before Bexsero - I hate to think how he would have coped with all that). The HV agreed with me that it was sensible to spread them out, even though officially the NHS wouldn't have considered that a medical reason to do so (I think you have to be pretty much hospitalised before the NHS will even consider that a vaccine reaction was a problem, but I digress).
I pretty much did as SideofFoot suggests and went to the appointment and only consented to one. For the others I phoned up the HV to arrange it, and then went along and waited at the clinic until she had a spare minute, so I wasn't wasting appointments. We could have waited for the recall, but the admin in our area is pretty inefficient, and we didn't want him exposed for that long. There were no issues with paperwork as he was due to receive them anyway - we just chose to do them separately. I don't think its that unusual tbh.
Personally, I think one of the major reasons that the schedule is arranged as it is, is for clinic convenience - as posters above have said extra appointments take time and money. The evidence against giving vaccines all together would have to be really strong to justify putting on extra clinics, and I don't think it's a question that is on many people's radar, so evidence is unlikely to be forthcoming soon. However, I also believe that spreading the vaccines out both minimises side effects and reduces the risk of needing calpol, which therefore increases effectiveness. (My research sources for this are mainly the JCVI, pubmed and vaccine clinical trial data, and I have a PhD in Biomedical Science plus a decade of post-doctoral experience, so I know how to read a paper.) I think the ideal scientific scenario to balance the need for disease coverage with reducing the number given at any one time would be to give a maximum of two at once, with things like Bexsero given at a separate sitting as the Calpol is known to reduce the effectiveness of all the vaccines given with it. Unfortunately, I think economic factors would scupper that though.