Making a quarter of a million pounds off an investment of about £400 in a gold mine is unrealistic representing over a 600 times multiple. Tulip Mania of the 1630's represents the biggest stock bubble ever and even this traded at a multiple of only 200. In reality the Hardacres could have already purchased an impressive family home with just their £400 invested in the gold shares.
Not the first time for the Hardacre name to feature in a period TV show, the iconic comedy series Brass had a similarly named central family four decades earlier in the 1980's.
At this time in Yorkshire only around 10% of people bought their own home, a small cottage costing just over £100, around £800k 130 years later, so comparatively property has cheapened despite the huge cost today. (Edit of note: in 1890 £100 would have been the equivalent of around £10,500 in today's money (2020's), not £800k).
A large civic building such as a town hall would have cost around £2000 to construct at this time, it therefore seems unlikely that the Hardacres could have earned this much, never mind enough for 125 town halls, in what appears to be a matter of weeks of months selling fried fish.