Further to the comments about the respective values of the Sinfield and Stewart homes, there is one scenario that is worth considering. It plays into the idea that both Helen and Ian were upscaling, not in terms of monetary value but in terms of sheer scale and style.
Both houses were sold at a slightly lower price than they were originally advertised (Helen's Lime House eventually changing hands for a little over £1,600,000), and this may have been because each was slightly "dated" in its way. Helen's was clearly the superior property, being in a highly desirable area of London, but it was modern without being "Grand Designs" contemporary, in an area where period properties are at a premium. Knowing the Royston area's housing stock, I would say the Stewart family home would be considered a rather nice house to raise a family in (and it's a in a lovely spot with a view towards a church tower), but barn conversions date very quickly, and the kitchen and brick chimney breast etc look tired. On a side note, I find it touching that the cushions are positioned in a similar way in each house, as though the same person (presumably Helen) had tarted things up for the estate agent's photographs.
If the couple did buy Hartwell Lodge together, as both of them - in fairness - seem to suggest, it is likely that they split the cost more or less down the middle - it seems to have cost them £1,100,000. IS may well have had no mortgage on the barn conversion, if he did have life insurance on his wife, and thus been able to put up to half a million in the "pot". If Helen paid roughly the other half, she would have had perhaps a million pounds change from the Highgate house, whereas IS would have had no capital remaining. So even if they jointly owned the house, she benefited from the sale of her original home in a way that he did not.
I'm mentioning this because of her concern that IS would be "financially vulnerable" if she died, and the financial provisions she made for him. It does seem that joint ownership of the house would make things feel equal at the point of moving in, but acutely divisive in the long term - basically, Helen could afford to live there and IS could not, if she was not there to support him.
Royston is quite an interesting town, property wise - it has some lovely housing stock, and is very well connected to London, but is SO much cheaper than nearby Cambridge, and even Saffron Walden, a similar market town a few miles away. In material terms, Helen was making quite a shrewd move by relocating there. Like a lot of newcomers from London, she suddenly had a stunning house and the security of a large sum of money in the bank that wasn't there before. ( I imagine that she might also have had some life insurance, not connected to a mortgage, on JS, simply because he died accidentally whilst they were on holiday) .
It is heartbreaking that this carefully planned new life didn't work out for her, and that she had no inkling of the horrors to unfold.