Congrats on your allotment!
I would say, if the seed spuds are well chitted, then why not get them in! Might not get monster spuds, but I can't see why you wouldn't get a very useable harvest.
And you are not to late with the squashes. Some folk on our site are only just putting out their plants, and I am going to sow some cukes and courgettes next week to try and lengthen the 'picking' season well in to the autumn, altho they will be under glass.