I am sorry to be the bearer of bad news, but Natwest are entitled to do this. A far as they are concerned, your wife owns 50% of the property and they are entitled to place a restriction on the property, as she is a registered proprietor, to protect their interest.
The charge will need to be dealt with in order for you to undertake any disposition of the property.
You may wish to contact Natwest to see if they will postpone their charge behind a new lender, however, if you are selling, it will need to be discharged upon completion of the sale.
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Unfortunately, this is correct. They can impose a restriction preventing you remortgaging also.
The only way you would be able to deal with this is by negotiating. Further, your new lender may not allow the charge which would mean that it would have to be discharged following draw down of funds.
If something was to happen to your wife, then yes they could enforce their charge against her share of the property.