I think possibly the biggest change I've noticed since finishing the work on the insulation is that the house is a lot more comfortable. The change in energy usage is there and over time will add up but the immediately noticeable effect has been on my living standard.
How much your rating goes up after the insulation work? And how long will you recoup your investment thru the (rising) energy bill?
I am in a rental with a low D rating at the moment.
I wonder if a low C would make a difference or have to jump to B.
However, it seems most properties , 1930, Victorian, 1990, double glazed, that I have viewed are all with low D / E
Only a few Edwardian Terrace are with a low C. ( from EPC report, they seem to have a cavity wall which give them a few more points to push them into C)
New built is B but significantly rare and must more expensive.
I bought the house before energy ratings were a thing, and won't need to get one unless I sell or rent out, so that's not happening. I did put internal wall insulation into a rental house that was single skin concrete block (it is a neighbouring property to mine that I was rescuing it from being a tear down) which with two new double glazed windows was enough to bring it up from EPC F to D. I still need to add to the loft insulation which should bring it up to C.
When I bought my house it was concrete block walls (the kind with hollow centres), two electric storage heaters and no insulation in the roof - I mean, none at all. The first thing to do was fifteen inches of insulation in the small loft. The first floor of the house is partially built into the roof so the next thing was insulating board between the sloping ceilings and the roof. A full renovation of the kitchen included a new insulated and electrically heated concrete slab where there used to be a suspended wood floor. The final work on the old house was external insulation. There was already uPVC double glazing but it was badly fitted with a tendency to get condensation and mold around the frames, the external insulation made a big difference with that and the existing double glazing is now working as it should. I've also put a single room extension on the house (previously just a two up two down) which is built to modern insulation standards and gets a lot of solar gain from south facing windows. The older part of the house is not quite up to that standard after all the renovations but pretty close, although it doesn't get quite the same solar gain. (There was a much more noticeable difference before the external insulation.) I'm not using heating yet and have the internal doors open between both parts of the house. There is one room in the older part of the house that is north facing, I'd probably light the stove if I were going to sit in there for long now.
I haven't added up the cost of all the work, it's taken place over the last 18 years and been funded out of income as I've gone along so I've never needed to - I might have to if I sold up because CGT will be payable on a few of those years and I've got the records if I need them. It has been thousands over multiple years, though. Insulating the roof was cheap, renovating the kitchen was not, external insulation was a few thousand. If you have the time and fitness it would be possible to DIY the insulation part of the external insulation and then get a specialist in to render over it.
I could add up my old energy bills but haven't got the usage records other than for the last year. The bills have gone down until just recently, although there have also been changes of circumstances involved - I kept the kitchen a lot warmer for my previous dog's old age than I do for the current young dog, for instance, so exact comparisons would be tricky. It's those sorts of changes in circumstances plus the change in comfort levels that make exact monetary comparisons and estimates of cost savings difficult.
Being in a rental makes it more difficult. If there is something relatively simple that could improve things, such as loft insulation or draft proofing, it might be worth talking to your landlord about it. I'm also a landlord and tend to reserve upgrades for periods between tenancies so as not to disturb the tenant or have to make tricky arrangements between tenant and trades, so for instance I've got a void coming up and will be using that to put in extra loft insulation rather than having to put a tenant to major inconvenience having it done around them. If a tenant came to me and said "can you get this relatively simple and cheap thing done, or can you buy the materials and let me do it" I would jump at that. But once the easy cheap things are done you are at the mercy of the rental market, I'm afraid. If I had a standard Victorian or Edwardian terrace and were looking at the long term then after putting in roof insulation and primary or secondary double glazing I would put in an air source heat pump with underfloor heating in an insulated concrete slab throughout the ground floor and think about insulating the walls (internal at the front, either internal or external at the back). It would be a significant investment and I don't think landlords can get grants or subsidies though, so until 2025 at the earliest when gas boilers start being phased out I can't see most landlords getting involved in that sort of work.