Hiya,
Oxygenators can be useful for your pond even if you do need to pull some out every so often. They also compete with the algae which will grow, so given the choice between thinning out a few plants or a bowl of pea green soup I recommend the oxygenators
Its worth getting a nitrate test if you plan on using the brook as a regular water source. If there is any farmland around you, then the nitrate levels in the water could rocket when they are spraying crops, and this would then give you algae soup again. Barley straw extract is supposed to help combat algae and blanket weed, but keeping an eye on the pond nitrate levels, and making sure that the pond gets some shade will probably help more.
If you can get hold of some Water Lettuce, its a nice plant to have both for using up some nitrates, and for providing some cover for any aquatic life which takes up residence in your pond. It probably wont survive the winter though, unless you move some in a bucket into the greenhouse.
I would suggest steering clear of bull rushes, as although they are stunning I have served many a customer with new pond liners when the old one was pierced by bull rush roots.
I also love the dwarf water lilies, again not native, but they do provide lots of benefits to the inhabitants of the pond, including somewhere for the frogs to hide should you get a visiting heron. A couple of pieces of piping laid in the deepest part of the pond would also help the amphibians hide from any potential predators.
Bog Arums and Marsh Betonys are worth looking at or Marsh Cinquefoil (Native) has striking flowers too. I always used Green Line for mail order but sadly they went into receivership at the end of last year. Stapely water gardens are usually ok (if a little expensive) as are Maidenhead Aquatics (which have loads of stores throughout the uk now) but we always had a limited stock at Maidenhead compared to Stapely.
My last recommendation would be to buy a bottle of dechlorinator, so if ever you do need to top up the pond with tap water, you can treat it first so as not to cause problems to the inhabitants.