The UK's vaccine advisory group (JCVI) aren't offering fourth doses to under 50s, unless you're in a clinically risk group or in a high risk job.
If you're under 50, without other risk factors, and have had a full course of vaccines (defined as the initial two shots + one booster) then your chances of serious illness (hospitalisation or worse) are so low that it's not seen as necessary. Adverse reactions can happen with any medication - including vaccines - so they only recommend medication when it's got a clear benefit.
For everyone, the evidence suggests that if you have had covid you should then wait at least three months (and preferably 6 if you're in good general health) before having a booster. Apparently your body creates a better/broader immune response in that situation.
They'll rerun the numbers if something changes (new variant or whatever) and they'll check them again every few months to see whether protection is waning in any of the groups.