Before the Human Rights Act 1998, you could not complain on European Convention on Human Rights grounds to domestic courts.
The UK as a state was signatory to and therefore bound by the Convention. However, public authorities in the UK were not required by domestic law to comply with the Convention. In some circumstances domestic courts could take into account the Convention but nevertheless they were not bound by it.
The fact that (generally) before making an application to the European Court of Human Rights you had to exhaust all available domestic processes just means that the state was entitled to the opportunity to make things right, there was some political leeway afforded to it, and the practical matter of cutting down the sheer number of cases to be heard by the European Court.
It is a bit like having to exhaust a regulated company's complaints process before complaining to the regulator. The regulator will tend to reject the complaint if the complainant hasn't been through the company's (reasonable) steps.
Among other things the Human Rights Act 1998 gave effect to the European Convention in Human Rights law. Now public authorities including the courts were bound by the Convention and complaints could be made domestically on Convention rights grounds.
Chapter 1 of the White Paper for the-then Human Rights Bill may be of interest:
Alternatively paragraphs 10-17 of The Government's Independent Review of the Human Rights Act https://publications.parliament.uk/pa/jt5802/jtselect/jtrights/89/8905.htm.