Here is the original ONS report
Their definition of long COVID appears to be certain ongoing symptoms after COVID. This definition does not necessarily correspond to the ME/CFS form of long COVID, unless the symptoms you have satisfy one of the diagnostic criteria for ME/CFS. For example, if you just have ongoing headaches, then that's not ME/CFS.
Also, even if you satisfy ME/CFS criteria, some long COVID patients recover after 6 months or so, thus this would be more like post-viral fatigue rather than long-term ME/CFS.
Here are the questions that ONS asked survey participants:
In a poll
I ran in one of the long COVID Facebook groups asking long COVID patients whether they have the ME/CFS form of long COVID, or just other forms, the poll results indicated that:
Types of Long COVID
46% had ME/CFS-like long COVID
20% had dysautonomia
(such as POTS, NMH, temperature intolerance, etc)
13% had post-viral fatigue
which had improved by at least 50% over time
6% had heart and lung damage
that had improved by at least 50% over time
3% had heart and lung damage
that had not improved over time
1% had silent hypoxia
(where blood oxygen saturation level go down to as low as 80%)
130 long COVID patients voted in the poll.
(Figures updated on 26 June 2022).
So this suggests that if there are 2 million in the UK with long COVID by the ONS questionnaire, then about 50% of those, ie 1 million, will have the ME/CFS form of long COVID.
That still quite a lot of people with ME/CFS long COVID, since before the pandemic, there were only around 130,000 ME/CFS patients in the UK.
At present, around 80% of the UK population have previously caught COVID, so that's 80% x 68 million = 54 million people in the UK who have been infected. And 1 million of those developed long COVID ME/CFS.
That means if you catch COVID, your chances of developing the ME/CFS form of long COVID form is roughly about 1 in 50.