Just had our house externally insulated. House is one of the Wimpey "No-fines" types, built during the 1960's. ("No-fines" is the term for the construction method, which is basically shuttered concrete with very little sand (or fine aggregate) used in the mix). Very strong & solid buildings, but thermally very poor.
House was completely scaffolded. They need dry weather for a couple of the rendering stages so it took best part of a month. Difference was noticeable as soon as the insulation boards went up. Rooms that were prone to damp from condensation dried out straight away. Basically two types of insulation boards, phenolic & polystyrene. Ours is 150mm polystyrene with a silicone render. The external system is ideal, as you basically insulate from out - in, & the inner surface of the wall no longer gets as cold. I have some info sheets on both internal & external on my works pc if interested.
The funding side is a bit of a minefield to be honest, but it boils down to paying upfront yourself, using Green Deal (which basically puts a surcharge on your electric meter), or ECO (Energy Company Obligation, which depends on circumstances but can be difficult to navigate the various rules, many of which keep changing). Top of the list for ECO are electrically heated homes, especially those deemed in need, low income, OAP's etc.
The process usually starts with an energy assessment to assess current effeciency, then a sliding scale of possible savings.
I'd recomend talking to your local council, as there are a lot of community energy groups picking up on it & the Energy Saving Co-Op are involved with sourcing installers & materials at discounts for bulk etc.
Hope this helps.