Do the underfloor first/early because you really won't be bothered after you've settled too much stuff in - having to move _everything_ out to get all the boards up is a major pain. And it has to come before kitchens at least. And in a draughty house it makes a big difference. It's very cheap, just a pain to get at.
Wall insulation (and airtightness) is extremely cost-effective too. And if you can do this early then you can size boilers and the like for the new, warm, low-energy house. In fact if you are fitting a woodburner and solar then you may find (like us) that the money spent on the boiler was pretty-much a waste as it never gets used (4 days last year - any old boiler will do at those usage levels).
Don't worry too much about needing spacetherm. Everyone worries about their rooms getting smaller, but trust me, you really won't be able to tell the difference afterwards. We put 40mm phenolic on the walls in previous (small, starter) house and it made no detectable difference to room size. We're putting 100mm on the walls in this (bigger) house and you still can't tell. Especially as you only need to do the 2 outside walls (maybe plus rear extension?). 2 systems to consider. PU/phenolic foam or Kingspan's rather nifty XPS battens plus dritherm. Latter has advantage of vapour-breathability if you are worried about damp external walls. Former is better U for thickness.
Loft insulation isn't in your list. Very cheap, make it insanely deep. At least 300mm.
Keep airtightness in mind the whole time - once you get to a reasonable insulation standard it becomes the largest heat-loss source. You need to detail things carefully as you go along, working out how say the UFI will connect to the IWI.
As you say the expensive part of this is windows. Good windows cost serious money. Think very hard about triple-glazing. The extra premium is rapidly reducing and windows are the worst U in the place by a large margin, _even_ if you get triples. Make sure the windows are fitted in an airtight manner. Window fitters understand _nothing_ about airtightness, and most have the wind whistling round the outside of the frame. Existing 10-15yr-old plastic DG is about U 2.3 to 2.0. Best current wooden DG is ~1.4. So you can spend a lot of money there for limited improvement. Probably rather less than properly blocking up all the drafts. Triple windows can get you down to 1.1 or even 0.7/0.8 but it gets _very_ expensive down there.
You can fit solar thermal to an existing DHW tank chite cheaply and easily with either a retro-coil or a PHE. We did that here as an interim measure before getting now tank. It's actually very good and isn't going to get upgraded for a while at this rate. The tank can then be upgraded whever you feel like it/have the money/it springs a leak. If changing the boiler then you need to decide the final system type (thermalstore/heat bank/vented cyclinder/unvented cyclinder) before going shopping because not all boilers match all setups (essentially vented/unvented choice). But you don't have to change it all at once.
If it was me I'd do something like: (and some guestimated DIY-cost figures - very ballpark)
1) loft insulation £200
2) underfloor insulation £300
3) gas heaters-> WBS £700-£2000
4=) Airtightness £100
4=) internal wall insulation (will take a while) £600
4=) solar thermal £700-£1000
(kitchen/bathroom/loo not before here - IWS and UFI has to happen first)
5) garden £100
6) boiler £700-£1200
7) tank £300-£1800
windows £2000-£4000
Although if boiler is old enough to have a pilot light that may push it up the list (6kWh/day for the pilot is normal).
Hope that helps somewhat
