First of all, unless you have specifically excluded the plants from the sale then they belong to the property and may not be removed.
However, at this time of year the only moving option is to put the plants in pots and plant them out when the soil is ready.

welcome to GoY.
yes I agree with Owdboggy. But if you can lift them then take as much soil as possible with the roots so you have limited damage. If you havent agreed to take plants then the best you can do is take cuttings of shrubs etc and take pot luck. for herbaceaous perenials then I suppose you could lift small pieces of the plant and pot it up. Or you could ask the new owner if you may take cuttings later in the year.

oh, I forgot to say, it is not a sale but a move from one church house to another - there were no plants here when I came and the diocese is happy for me to take them

Full steam ahead then. chose a time when the soil isnt frozen though. If they are staying in pots for a while mix some soil with compost too to help them push roots through.

Fair enough then. Only mentioned it as people seem not to know that it is illegal to remove the garden when the move!
As SBG says, try to dig things up when the soil is not frozen and with as much soil on the roots as you can. If they are evergreen things then wrap the top growth in fleece to stop them losing too much moisture whilst in pots.