Resale of a House with Estate Rentcharge

Made an offer on a c. 40 yr. old freehold house in London. Had offer accepted.
When going through the docs as part of pre sign-off due-dilligence, turned out there is a 300 GBP / year Estate Rentcharge with draconian remedies like:

• Right of re-entry to take stuff.
• Grant lease of the house/land.
• Sale or mortgage of the house/land.

Two questions:

• Does this mean the house is unsellable?
• If I get these remedies removed, though rentcharge fee remains, does the house become normal and easily sellable again?