I notice some people have to stay online during a power cut. You may be a communications professional, have some other business or family need or just be an addict. When there is an area based, demand-management power cut (see my previous article on this) mobile internet seems to be the only option as landline or fibre-broadband-types all need mains power. But some mobile masts won't work as not all masts are required to have back up power.
To maximise your chances of staying online during a blackout you need a device that can pick up a mobile signal from a mast further away and a way to power it. Your mobile phone might well work, but running a mobile as say a family hotspot drains the battery fast.
Three options occur to me from my long experience of rural broadband and living somewhere with a flakey power supply.
The first is a portable MiFi-type device - a pocket sized mobile router usually sold by a mobile company but there are loads on Amazon for which you provide your own Sim. These devices create a small mobile WiFi hotspot. The downside is a relatively small antenna (though perhaps bigger than a phone) which isn't amazingly sensitive to weak signals but the upside is that they are designed to be charged from a regular USB outlet so you can keep them running from a small portable power bank.
If you are wondering what Sim to buy - my hunch (and it is only that) is that in the UK the ee network should stay up the best as they provide comms to the emergency service using their 4G system so you would hope they have back up power supplies to their masts.
A second, more powerful (and expensive) option is to use a regular type of router that has a 4G modem built in and plug that into a larger power bank (picture or your car's cigarette lighter). They only need maybe 18W so you should get many hours of power. My preferred 4G Router is in the excellent TP-Link Archer range, which has several models that take a 4G sim. It looks like a regular router but you just stick a SIM in it, the router then logs on to 4G/3G and creates a wireless hotspot. The bunny ear 4G antennas are much larger than those in your phone and should pick up signals from masts further away if the one near you isn't running (this has been my experience in use). You can plug the router into a larger power banks’ 3-pin socket. In the pic above I am getting wifi with 25Mb/s up and down with a 40ms ping - perfectly usable in an emergency. You can make an Archer Router even better at picking up distant 4G signals with a simple external antenna like this. You just unscrew the bunny ears and screw the antenna leads on, then stick the antenna out of an upstairs window, you don't really need to attache the antenna to anything for infrequent use. This antenna would often add a couple of bars to the signal and even work well indoors.
There is another power supply option for some 4G routers - the models I use in the Archer range require a 12V power supply, the same voltage output by the cigarette lighter in your car. Larger power banks tend to have this sort of 12v socket. With the right adapter lead you could plug straight into that, freeing the 3-pin socket for something else - this is what I am doing with the power bank in the pic above as the power bank has a 12V socket for camping and in car gadgets. It is also more power-efficient to plug it into 12v as you skip the need for a second transformer. You need a lead though, which the manufacturer often doesn't supply. To find a 12V car plug lead for a 12V 4G router go here and find one with the same plug tip dimensions as your router requires and that can manage the current. To find the plug tip size for your device first try searching for a replacement regular power supply for the router model number - the after market power supplies usually include the size of the plug tip in their specifications.
A third internet option for some is to check if your car has wifi built in - modern models often do, though it can be a pain to set up and you might need to buy a SIM to fit in the car. Then you can drive around hunting out a signal while trying to avoid comparisons to the (profane, controversial) South Park Over Logging episode.