No government can shift in January from a high-risk-only vaccination plan to a compulsory one. The amount of vaccine available is based on what’s ordered the previous Spring (less any production failures). If you ordered less and want more you’re generally out of luck. They can declare an epidemic or pandemic whenever they want, and they can probably squeeze a few additional vaccinations out of any extra supply that might otherwise have gone to waste, but that’s about it. The doses available are a matter of policy based on long-range predictions, and cannot be adjusted upward significantly.
The US has an awkward history with compulsory flu vaccination. In 1976 the swine flu was forecast to be horrific, on a par with the Spanish flu catastrophe in 1918. A more-or-less compulsory vaccination program was undertaken. Some serious side effects (500 cases of Guillain-Barre syndrome) made headlines and were politically costly; additionally, the flu pandemic never materialized. Ultimately 24% of the US population got vaccinated before the program fizzled out. Gerald Ford did not serve another term as president. I don’t think another compulsory program is likely.
Whether the current situation is serious or not is partly a matter of personal perspective. I have made no representation about the prevalence in the UK or the US. The numbers, however, are the numbers, and whether your personal circle has gotten sick yet is generally an unreliable way of counting.