Chances are mate, that the vast majoriity of excess dementia deaths from April were directly caused by COVID infections. At that time, COVID was ripping through the elderly who weren't getting tested, so the general consensus among medics is that the cause of death was being missed.
When you look at the equivalent figues for May and June, one month was a bit below average for dementia, and the other month was a bit above average. If dementia deaths were being caused because of lockdown, including things like lack of visitors, then you'd have expected the April trend to have continued.
It's impossible to say for sure, and I doubt we'll ever know with any degree of certainty, but it's more likely than not that thousands of elderly people with dementia died in April as a direct result of them being infected with COVID.