The Spirit class were never used on the Western Channel, though there was a crazy idea to use the Pride of Bruges on a Portsmouth - Channel Islands route in a P&O bid to take over from Condor. You would have been on the Pride of Cherbourg or Hampshire, ex Viking Venturer /Valiant from Townsend Thoresen days, seriously ugly ships with a brutalist resemblance to the Spirit class, and as you say, we'll past their best.
Not strictly true. Each incident contributed hugely to safety in different ways. The Herald disaster led to the very rapid implementation of the ISM code, designed to reduce human /procedure failings - effectively making quality assurance mandatory on all international ships. This was the biggest change in shipboard safety since the Titanic (which lead to the introduction of SOLAS). It was designed to prevent, for example, a bow door being left open by accident, with closed loop reporting and checklists, and to improve responsibility in the office, so shipboard concerns could no longer be ignored, both massive issues at Townsend.
Physical changes to the ships were limited, mainly things like sensors, to ensure that a similar incident couldn't happen again.
The Estonia was a design issue with the ship, rather than a procedural issue - like most ro-ro ferries at the time she was not designed to cope with large amounts of water on the car deck. The Stockholm convention rapidly followed her casualty, leading to sponsons, watertight doors and other features to allow ships to survive an intake of water on the car deck.
In hindsight, it was obviously a great shame that physical design changes didn't follow the Herald, but the changes that did happen were massive, and have saved lives across the whole shipping industry, whereas the Estonia's changes only affected ferries. Even if they had been made, the relatively short time between the two incidents would probably not have been enough to make the legislation and modify every ship, so it may not have saved her.