Hill work and interval training are a good idea. How about setting yourself a different goal such as increasing your distance to 10km? Sometimes an increase in distance can lead to an improvement in speed.
Well done on what you have achieved so far, I love to be running those times for 5km!
A shorter pool gives you more practice at turning however you will need to get used to the feel of the 25m pool. Sometimes leisure centre pools get very busy and it is nigh on impossible to train in them though! You can't win either way, what you can do however is work on your stroke technique and try to build up some stamina no matter what the length of the pool is.