Why a stronger empire would not simply accelerate Type 21 and Type 42

Would a stronger British Empire bring Sea Dart, Sea Wolf, Type 21, Type 42, and Type 82 into service sooner?

Not automatically. A different fleet program changes the requirements, and might eliminate a class such as Type 21 instead of merely accelerating it.

A wealthier or more enduring empire would change the requirements behind historical programs as well as their timing. A larger continuous frigate and destroyer building plan could remove the shortage that produced the Type 21 requirement altogether. Sea Dart, Sea Wolf, Type 82, and Type 42 might appear on similar or different schedules depending on strategy, industry, and the ships already in service. The counterfactual must therefore rebuild the procurement logic rather than move every familiar class to an earlier date.