The difficulty in shielding the most vulnerable as the virus spreads through the general populace is one of the great flaws with that "herd immunity" approach. You'd be relying on the Tories doing something they have already comprehensively failed to do. Just as Sweden were also looking at protecting care homes and failed to do so. Because, as you say, it's an extremely difficult thing to do. It's like setting a wildfire and then saying "well this is a good idea as long as people indoors don't get burned to death". You still have to do something to actually stop those people from getting burned as the fire rages around them.
So if government were pursuing that herd immunity strategy, the first solution to try and protect those in care homes would be not to pursue that herd immunity strategy. Because a virus that is suppressed in the general community will have a harder time repeatedly getting to the most vulnerable than a virus that is buring through the general community. And even countries that do well in suppressing the virus have and will struggle to protect their most vulnerable.
Beyond that, you would start hunting the virus in those facilities by mass testing everyone, whether they're showing symptoms or not, as other countries have done. When people say "all a lockdown does is buy you time", that's the sort of measure you can implement with that bought time. As is it will apparently be June at best before that happens in the UK.
As it happens though I don't think the UK have been perusing that deliberate herd immunity strategy. Rather they've found themselves caught between different approaches and been ineffective as a result, which unfortunately doesn't then relieve the need to lift lockdown.