Building regs do not apply to conservatories.... well, thats what our building inspector tells us.. they only come under regulations if put on the same time as an extension, or when the house is built. And you can plumb in a radiator.. we have done so several times... even when one was required with building regs.... there is nothing stopping you from using your existing central heating APPART from if your boiler is capable of taking another radiator....
in the summer, if the thing has a glass roof or polycarb.. you will cook to death in it, so the split system may be a good idea if you have room for the box on the outside as well....
To point out, it does depend on what your conservatory is made of, and what type of glass is used.... again, as conservatories 99% of the time do not come under building regs... people tend to go for the cheaper option of double glazed units instead of using K glass... K-glass does have a good thermal rating, hence why all new build houses have to have K glass installed.... but thats when things get complicated as you can also get climateguard glass, which is supposed to stop too much heat getting through the glass from the outside in for summertime.. again expensive as hell... (we built a conservatory with over £3000 worth of glass in the roof alone) same with the roof, if you have polycarb, then that will lose heat, if its a glass roof, its all down to if its K or not.. probably not, as its expensive as fook to glaze a conservatory roof with K glass, as it will have to be toughened glass to begin with so looking at major bucks to do that.
A major factor will be is it open to the rest of the house all the time ? or something thats only going to be used every once in a while ?
Electrical heating is the easiest way to go.. as its quick to heat up a room as well, but can be costly in the long run.. but you will have to make sure you get a heater of the correct rating, and if its a big one, make sure it doesnt exceed your house fuse rating (very unlikely, but not impossible to go past it once you add up everything in the house)
As pointed out by several people, air con is a must if you want to use the thing in the summer... gets VERY warm in there.. built a conservatory last summer, glass roof.. still work to do after the glass all went in, so had to suffer working in 40C + temperatures doing heavy work...
You may have to get an engineer to install the air con, as even the 'DIY' kits have major drawbacks.. we are currently stuck with some usints that have gone U/S.. 1 years guarantee.. phoned manufacturer, but they said even though they are marketed as DIY kits you still need to get them tested for any leaks in the system, as they eventualy break if they leak.. specialist equipment required for that testing....
underfloor heating can be very good.. but has one massive drawback.... if something goes wrong with it, you may end up ripping your floor up to get at it... if its electrical, thats probably not a major problem, but if its into the central heating system.. you will have to do somehting about that...
just a quick note, sometimes you do need planning permission for conservatories, and thus they come under building regs.. this is simply down to their size, as different councils have different area rules for what needs planning permission in this regard...