It's not been established that the purpose of re-mortgaging was to buy the French land and property, which is estimated at £20-35k when bought. According to a local quoted it was inhabitable at time of purchase, 2007. That date aligns with the period SW was stealing from the Hemmings and renovating her Welsh farmhouse with slate floors and a powder-blue aga.
SW seems to have had a spending addiction, Hemmings mentions she paid off her shopping debts with forged cheques. And may or may not have taken out 4 credit cards in TW's name. There may be other financial outgoings (even perhaps a property portfolio investment, as that gets mentioned so much and usually there is some twisted tie with reality), they also both lost their jobs, SW in 2008, TW sometime before 2013.
Expect the remortgage became necessary somewhere along their financial 'path' without necessarily being directly linked to the desire whim to buy the French place.
From the frequent references to never being able to holiday abroad while younger it was a 'we deserve this' holiday home purchase and or an 'if all else fails' backup home option but they didn't realise how much work it needed, or TW health deteriorated before they could do it up. If the latter is so, that was prior to the Haye Cider Farm stay and would seem to indicate they wouldn't have been able to renovate that (though personally I think that property was always in good condition).
The thing is, we're still hung up on fairly minor details and taking them at their word a bit too much, as is instinctual: so understandable. Maybe everything took a downward spiral in their 40s, but remember these individuals could have lifelong form at it all. They don't exactly make master criminals but wouldn't be surprised if they've previously conned and stolen from many others.