what about other shed suppliers, screwfix.com for example if funds are limited, they supply sheds at very competitive prices, or another local shed supplier that can make something up to your specifications ?
i moved into a house 4 yrs ago that needed alot of renovation, the first thing i did was build a huge shed in the back garden - to store equipment in when renovating the house, i also knew when the house was done this would become my modelmaking studio, it has a fullsize solid wood front door and 4 double glazed leaded windows, The base is concrete as i knew that one day i would bolt down my circular saw and lathe ! the framework is all timber made as a frame in 2 by 4 timber, it then had double glazed windows fitted ( these sealed unit windows came from a house where they were being removed as they were building an extension- i picked up the door and window windows off ebay for £20 ) but as i was making the timber frame i could make the frames to suit the windows i bought !
the shed is insulated and lined out on the inside with timber board so it is snug in the winter,- however with power installed i have a heater too !
i also have a little beer fridge
and my tools and two 8 ft benches, the shed would be around 10 feet wide and about 24 ft long - even with all the materials and the windows and door i think the cost of building it was no more that £ 420 for materials including the electrical consumer unit and RCD. but i did take 4 full weekends !
it is worth considering that if you want a custom home for your layout... ( and lets be honest your layout is worth more that £420 ) then maybe it is worth investing some time and building your own shed/ building that fits your needs exactly....
buying maybe the easy option but if you then have to spend time ' modifying' it then maybe its worth doing it all from scratch...plus if you buy lengths of material and stuff from a builders merchants and construct your own then its often cheaper in the long run than buying a pre made shed kit.- maybe you can call in some favours of f mates to give you a hand in the build ?
anyway whatever you decide to do - good luck !