More British grandparents here! Yes, the insurmountable barrier at the moment is the US Presidential Proclamation (Jan 2021) forbidding travel to the US by non-US citizens from either UK or Schengen countries.
Our grandchildren in Colorado are 5 & 3 now, and we haven’t seen them (or our son and daughter-in-law) for very nearly 2 years. If it were only a matter of paying for extra tests, and quarantining on the return to the UK, that would be a price we’d be happy to pay... But there's no getting round the US ban!
As you might expect, we've been monitoring the situation as closely as we can. There's a frustrating amount of opinion out there, but an absolute lack of concrete facts. The latest opinions that I've read are that the proclamation might be lifted in early August, at the very earliest. The UK has been doing well on the vaccination front (I've been fully vaccinated for over 2 months now), but infection rates from the new delta variant are growing rather alarmingly, so I'm not feeling very optimistic about being able to travel before the end of the summer at the earliest.