They are actually not dormant seeds but embryos which have actually photosynthesis all the time. They put out the sucker when spring turns into summer or even late summer. First leafs appear when they have established some growth beneath the bark.
I had young plants which lost their first set of leafs. Some made another one the following year. Older ones loosing every green part above the bark came back after one or even more years, from the part beneath the bark. They had usually four or fife sets of leafs at once just above the swollen bark.
England is not too cold for mistletoe in the north, but maybe the days are too short in the winter for the seedlings, or another reason I don’t know (birds?).
http://mistletoe.org.uk/homewp/wp-content/uploads/2012/07/infosheet_1_Distribution.pdf
The continental climate of Europe, where they grow, has much colder winter than Scotland which is heated by the golf stream.
The species found on apple grows here in some areas on very unlikely trees like Betula (near Berlin), Sorbus, Robinia and some Acer, rarely Oak abundant most places on Tilia, Populus and Salix. Here there are great areas where there are non to be found and I really dont know why.