Most of the budget portable generators (which won't drive the neighbours up the wall with the noise) are around 1-2kW so unlikely to support peak useage if you cook on electric.
The aim of batteries would be to keep essential services running - fridge/ freezer and central heating pump. In fact, configured as a UPS this would remove any issues about reconnecting or a switch over device. Just keep your essential load on batteries 24/7 and charge when the power is on.
So day load would be much like the overnight load assuming you cook on a portable gas stove (£10 at the moment) and use candles and such like for most of your lighting.
Apart from the fridge/freezer and the central heating you could put in some 12V lighting and avoid using the inverter for most tasks.
Still, if we ignore the silent(ish) portable handbags mainly used for camping, then looks a good bet at £1100.
Looking further at the portable handbags they don't have much of a fuel tank so they won't run for more than about 6 hours so if you want to use them overnight you may well have to get up and refill them.
I don't think they are designed for continuous running, either, so if the expectation is that power will be off for more than a day they may not be suitable.
Could be the time to start stockpiling old Lister genny sets which became redundant when rural places came on grid. :-)
So I guess the main question is "What load do you aim to support and for how long".
You also have to factor in the cost of automatic switching to the backup power supply.
Not being a champion for a UPS configuration in the house, but more interested in how well it costs in compared with various other options.
For a normal house in normal times this doesn't seem cost effective, but with long term dodginess in power supplies this might an interesting prospect.
If you had your basic IT kit (modems and routers and PCs) plus fridge and freezer and (non-electric) CH on a UPS then you could keep essential services up over a relatively long period of power in-and-outage.
You might even consider that solar power could have additional benefits once you had invested in storage technology.
From what has been said so far I think that full off grid independence probably costs about £1,500 - £1,100 for a diesel generator and £400 for the switching.
You might still want your PC(s) on a UPS, though.
Cheers
Dave R