There are a few options available:
Residential contaminated land indemnity insurance protects you from a remediation notice's cost implications and can reduce the risk factor.
Search for a lender with flexible criteria around contaminated land that will consider the application provided there are safeguards in place.
Source a specialist mortgage provider who may consider land mortgages with oversight of the remediation plan.
Another option could be to look at a bridging loan for the land purchase, including financing for remediation work costs. When the contamination is removed, it should then be possible to remortgage on a long-term product at regular rates.