https://en.wikipedia.org/wiki/Primitive_accumulation_of_capi...
The name Van Allen implies Dutch ancestry and, according to the article, he has a Yorkshire accent, so it sounds very implausible that a public park in London was "stolen" from his ancestors.
He has no more right to deprive other people of the use of this land, by building a private residence, than anybody else.
The solution to homelessness in London is through government building public housing to replace all housing stock they sold into private ownership not from individuals nabbing their own bit of a public park.
We could back that up with better provision of the mental health services that have been de-funded over time.
Otherwise this just looks like an individual trying to take something from public ownership into private ownership on the sly. Like a land owner trying to fence off a public footpath.
[0] https://www.heathandhampstead.org.uk/home/origins/
[1] https://www.legislation.gov.uk/ukla/Vict/34-35/77/enacted