A combination of lies and the information advantage the banks / building societies had over normal people - even those well educated. They were used to being trusted back then and took advantage.
Once they were found out they knew they were caught with no way out for being sleazy scumbags. They didn't attempt to fight the recompense scheme at all ... which speaks volumes. Reassuringly expensive lawyers could have delayed putting things right for years. They would have lost eventually regardless: This hit the property owning middle classes who are key swing voters. Punishing evil bankers would be labour's wet dream under Kinnock and Foot and the tories lack of spine post maggie would have them running for cover.
What's depressing is they thought they got off lightly and got greedy in other markets, which lead to the 2009 financial crash. A bunch of dead wood got cleared out, only for their replacements to stuff up in a new and exciting way