OP, I really feel for you and would be inclined to reduce my offer on the property based on having assumed at the time of offering that those items were included.....
We completed on our latest sale and purchase in December and had a few hairy moments where we weren't exactly sure what was included with our purchase. The vendor had moved out to his new house a year or so ago and the house was empty, but when we viewed (150 miles from our then home) the Aga was on and the house was toasty. We were shown over by the EA who insisted the Aga was included - as you'd assume - but the printed blurb/RM details said that it was available by separate negotiation.
The fixtures and contents form omitted to mention it altogether (but as English wasn't the vendor's first language, some of his answers were a little indecipherable) and despite our solicitor requesting confirmation several times that it was to stay, the vendor's solicitor was reticent in responding.....right up till a couple of days prior to exchange, when he finally confirmed that it was included.....grrrr!
We couldn't imagine someone - especially a guy that had bought himself a new pad for close on £2m - would be arsed to remove an Aga, but you never know.
Imagine our surprise though, when the day before completion our solicitor called to say the vendor had realised the (free-standing) washing machine was no longer working and was offering us £350 for a new one.......when we were actually taking our own with us anyway - which we didn't mention of course!
He obviously realised he'd included the washing machine on the F&C form and that we'd be expecting it to work.....so there are some nice, honest people around still after all!
On reflection, as the OP's vendors are emigrating, I'd be seriously inclined to call their bluff as the chance of them taking those items overseas is pretty unlikely imho......