Importance of maintaining low prevalence
Although vaccination of most vulnerable groups will have reduced the proportion of community infections that lead to hospitalisation and death, there remain many advantages from an epidemiological perspective in maintaining both low prevalence and R<1. It makes it easier to prevent a return to rapid growth in the epidemic which could lead to the NHS being overwhelmed (e.g. because it gives more time to react to increases when starting from a low baseline, it is easier to spot outbreaks in advance of them growing large, and Test Trace and Isolate (TTI) can be more effective at lower prevalence). This has been shown in some countries that have very low or near-zero Covid-19, since occasional outbreaks can then be dealt with quickly, including rapid sequencing of all cases to search for new variants. Lower transmission also reduces the in-country risk of the emergence of variants of concern as well as slowing spread of any VoCs (including imported VoCs). Lower infection rates will also reduce impact of post-Covid syndromes and allow more NHS capacity to be used for routine care. Since groups from a lower socioeconomic position and minority ethnic backgrounds have higher risk of infection and lower vaccination rates then any increase in prevalence is also likely to increase health inequalities in Covid-related illness and death.
There is significant risk in allowing prevalence to rise, even if hospitalisations and deaths are kept low by vaccination. If it were necessary to reduce prevalence to low levels again (e.g., VoC become more pathogenic for others previously less affected), then restrictive measures would be required for much longer.