He may have been a liar, but we don't know whether or not he lied about this.
Logic should tell you, MrsWah, that June and Nevill would never have even mentioned having the twins adopted or fostered. Reasons why not:
Colin had custody
It wasn’t up to June or Nevill who had custody of the boys
June would have felt huge shame having her grandchildren fostered out/adopted
June and Nevill adored the grandchildren so why would they want them removed from the family?
Colin would never have agreed
Sheila would never have agreed
Sheila had joint custody
The twins would have been traumatised
June and Nevill had enough money to employ nannies, au pairs, housekeepers...so no need for fostering/adoption
The twins had a home in London with Sheila and with Colin
Had the worst happened and neither Sheila or Colin could care for the twins (which wasn’t the case — Colin was an excellent father who had the support of his parents’ too, who also adored the boys) June and Nevill would’ve loved to have had the twins living with them
The whole thing is utter nonsense