It is important that you consider avoiding, minimising and compensating your losses, following the mitigation hierarchy, and then consider off-site units.
Only units from gain sites that have been registered with Natural England count towards biodiversity net gain. A "unit" that does not come from a habitat already registered with Natural England has no impact on your BNG, under the Environment Act.
When you are looking at off-site units, you should also be aware that:
Depending on where the seller's gain site is located, 1 unit from it may be worth less than 1 unit to your development. This is called the spatial modifier. Make sure when you buy units that you are happy you are buying the right number, once the spatial modifier is applied.
Where you have habitat losses, the units you buy will need to be a valid trade under BNG rules. You should satisfy yourself that your supplier is selling you appropriate units given your losses.