> connect to the house central heating
I was thinking on that. Sadly I think in R-factor, which is BTU and feet (of a English king), pounds, and degree F. None of these very-British measures are used in England today.
The walls, without stuffing, are about R-5. Glass fuzz stuffing could make it R-8. This is half of the minimum in modern US homes (3.5" of fuzz makes nominal R-13). The roof appears to be inch boards, so R-1. The floor is concrete so less than R-1 to the ground, which will warm if heated 24/7, but will cool to weather average if heat is occasional.
I'll say it is 12'x8'. 280 sf of walls, 100sf of roof or floor.
Divide sf by R to get an equivalent square-foot at R-1 (reference).
280sf/5 = 56
100sf/1 = 100
100sf/1 = 100
The walls are the least of the loss. Carpet over padding, and 2" foam between the rafters, would get floor loss to 50sf and ceiling loss to 10sf. Total 120 square feet of loss re:R-1.
The magic of R is that (sf/R)*T (temperature rise) gives BTU. Which now means nothing in the country which invented the BTU, but is how we do it here.
How cold will it get? Wikipedia lacks data for Norfolk but Cambridge is close. In my degrees F, he has average 45 F Dec-Feb. Actually in my Maine workshop, 45 is workable (specially when it is 20F/-7C outside at noon). Englishmen may call 60 F a cosy temperature. So 15 deg F rise with 120sf of loss is 1,800 BTU. This is only a 600 Watt heater. Indeed a 16-channel tube console could be the main heat. If he wants 70F, 3,000BTU. Here the next step would be a 5,000 BTU gas heater, which could be bottle-gas for occasional use (safety codes permitting).
He will have nights to 1F/-17C. To hold 60F inside is 7,200BTU or 2,400 Watts. A large electric fire, and a large electric bill to do that Jan-Feb.
If, as I understand, english homes heat with electric hot water, it makes no sense to run water pipes out through the cold lawn; a 10 Amp electric feed is much cheaper and somewhat more efficient. If he gets gas or oil much cheaper than electric, and heats many hours a year, then central heat loops may make sense. (Disuse in freezing weather invites trouble.)
Me, I'd have built a heated workshop ON the house, so a hot-air duct could be brought right in. But Maine is colder longer. And his neighborhood may have lot restrictions/traditions which ban the usual Maine lean-to shed but tolerate a hoe-shed behind the garden.