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.