The United Kingdom is divided into eight swimming regions and they all have their own website as listed on the swimming org website.
You must participate in the region your club belongs to. Your club will know which region it is.
Each region sets their won participation levels and entry times for their regional events. Regional events are also organised into age groups. As these are normally faster than County events, the age band sometimes starts at age 14 and over but recently younger swimmers can also take part as long as they are a minimum of 9 years and meet the required entry time. The entry time must have been achieved in a licensed meet and recorded on the ASA webiste within 12 months of the competition/entry date.
Each region publishes their events well in advance and entry packs are posted on the respective websites. Normally swimmers enter via their home club that coordinate the entry of all eligible swimmers and make a swimming coach(es) available to supervise their swimmers.
Regional events may be in a town central to the region.
Contact your club if you wish to participate.