Header tank could be near ceiling level in house if no loft. Have a look at my recent post in the Hall of Shame on this website. That was a plastic tank so not what you'd want and installed by someone with autism who didn't care what it looked like, but it gives you an idea what can be possible if you think outside the box. Is your house single story or just lacking a loft, I wonder?
Oil-filled rads: I had one in a caravan once. It was portable and felt and looked nice but...
It was a DeLongi portable one and rated at 2kW. However, while the heating element was 2kW, I suspect that the actual heat exchange from the radiator to the air was less than 2kW so the heating element used to have to cycle on and off to avoid overheating the oil, and the heat output to room was not as high as the 2kW anyone would have reasonably assumed. Which? reports have found this as well, so it's not just me. (For fast heat, this sort of thing
works as well as anything:
Tesco direct: 2000W Convector Heater with Turbo Boost and should kick out the full 2kW and if you can turn down the heat output to a trickle once the room is warm, it's a practical, if unelegant solution.)
Also, the thermostatic control was pants as the thermostat was mounted on the unit itself and so wasn't able to monitor the air temperature with any degree of consistency whatsoever as it got influenced by the heat coming off the radiator.
The nice thing about it is that when it went off it stayed warm for some time so you didn't get the on/off yoyo effect you can get from, say, fan heaters, or the Tesco thing mentioned above..
Quite honestly, I feel oil filled rads haven't much to offer other than the fact that they don't look and feel as cheap as cheap wound-wire convector heaters, or infra-red heaters. The efficiency of ANY electric heater (except heat pumps) is identical, so it's what suits you best, really.
You can get some very posh wall-mounted radiant heaters these days that look amazing but have quite low heat outputs so probably best for really well insulated buildings.
It's worth noting that what we perceive as warmth is only made up of 1/3 air temperature, with the rest being the mean radiant temperature (i.e. the surface temperature of the objects and surfaces around us), so that's the advantage of radiant heat. That said, I've got underfloor heating at home and I can't honestly say that it's made such a difference that I don't know how I lived without it. The associated draughtproofing and insulation, though, has made a tremendous difference.