The difference lies in when the test must be performed.
A PCR test can be used for departure or outbound testing for those countries that require a PCR test instead of a cheaper and simpler rapid antigen test.
A Day 2 PCR test is required after return from Green List countries. This is used to help detect any variants that may be entering the country and asymptomatic carriers of the virus.
The PCR Day 2 & Day 8 tests are sold as a single unit as the lot numbers are registered together and must be performed by the same person. The Day 2 test helps to detect variants and the Day 8 test is to check whether someone has Covid-19 towards the end of the incubation period. If you test positive on Day 8, you will need to continue self-isolation. Day 2 & 8 tests are mandatory for anyone entering the UK from an Amber or Red Listed Country.
A Day 5 PCR (Test-to-Release) is not a mandatory test but can be used as part of the Test-to-Release scheme for travellers returning from Amber Listed countries. The test can only be delivered to your house on day 5 (with the day you return being day 0), of quarantine and performed on Day 5 of quarantine. You must remain in quarantine until a negative result has been received.