One of the arguments in favour of abusing the Web PKI to do other things is that tooling for the Web PKI is in a relatively good place. And a lot of other things that you might ideally hope exist actually DO exist for the Web PKI. There's independent oversight, somebody is actually reading those yawn-making audit reports, it isn't perfect but it's not just make believe either.
But on the other hand, the Web PKI is ours, and so if it suits us to change it we don't really care that this is annoying for your payment card systems, jet aeroplanes, nuclear submarines or whatever else you've duct-taped the Web PKI into. This was fun to watch with SHA-1 for example and is currently causing some fallout for names with underscores in (DNS can handle a name with an underscore in it but they're prohibited for hostnames. Lots of people ignored that, but that's their problem, not ours and they are not happy about that).
Can't seem to find anything on web.
On underscores: https://www.digicert.com/blog/digicert-pushes-underscore-ext...
On SHA-1: https://blog.mozilla.org/security/2015/10/20/continuing-to-p...
Part of the reason to do this is that if CAs issue the certificates then their customers buy them, and create pressure to accept them. If none of the CAs are willing to issue this never comes up.
Another is "unknown unknowns". Security systems don't play well with undefined behaviour, if we explicitly forbid everything we don't want to exist that minimises the risk of such undefined behaviour anywhere in the certificate handling code getting exploited. It's a defence in depth.
 (link in Vietnamese) https://congnghe.tuoitre.vn/ericsson-xin-loi-ve-su-co-sap-nh...
It's a very clever strategy, nonetheless.
O2 have 25MM direct customers, plus another 7MM through reseller networks — so that's 32MM people directly affected by the outage.
Presuming O2 receives an average of £30 per customer per month (which I think is a reasonable estimate), that's £940MM/month, working out to £32MM per day in a 30-day month.
O2 are refunding customers 2 days' worth of service for the outage, so that would be £64MM in lost revenue.
Add on top of that brand and reputation damage — and other factors I've no doubt forgotten to include — and £100MM doesn't seem unreasonable.
Also average monthly can't be so high maybe you're including the device cost as well.
Remember that many might have prepaid and no contracts.
The plural part of your definition here is not really relevant.. i.e. 10M = ten thousand, 10MM = ten million, 1M = 1 thousand (not 1 million, singular)
Most people colloquially seem to use k for 'thousand' and most people would probably intuit M as 'million', but I think people still go with MM for avoidance of doubt.
10M, ten thousand
10MM, ten million
10MMM, ten billion
here in sec. 3: https://www.druide.com/fr/enquetes/abréviation-de-million-et...
Still I suppose it's better than sussing out whether someone is British and using "long" billions.
(in most of the world, "billion" = 10 to the 9th power; in older British usage still sometime seen, "billion" = 10 to the 12th power, and 10^9 is "thousand million")
Once you understand this system you might also understand why the long system makes more sense.
1000 = Mille M
1000 000 = Million MM
1000 000 000 = Milliard MM M
1000 000 000 000 = Billion MM MM
1000 000 000 000 000 = Billiard MM MM M
Britain does not use "long" billions. Not since the 1970s or so. Now days, billion always means 1000 million.
From the Guardian style guide:
"million - in copy use m for sums of money, units or inanimate objects: £10m, 45m tonnes of coal, 30m doses of vaccine; but million for people or animals: 1 million people, 23 million rabbits, etc; use m in headlines"
I doubt they would be compensating payment plans for devices.
* Two days of an airtime contract (so e.g. if you just pay them £30 per month, that's about £2 in value for you)
* 10% discount on two types of pay-as-you-go payments with no specific limit but expect one to be announced.
The latter in particular you can expect to be heavily abused, because O2 needs the PR. I'd be astonished if that limit is any lower than £100, meaning they're giving away a £10 value.
Puts into perspective how many customers don't change their plans when they're able to...
e.g. Transport for London's dot matrix displays could not show bus times for the 14 or so hours the network was down, apparently iPads issued to NHS staff to manage patient reports used O2 sim cards so that couldn't work either. Probably a whole range of small to medium enterprises affected too.