So Brettonians. They are considered near the bottom of the heap. It's pretty much true, sadly enough. They are an army without crazy magic phases, and without really strong shooting phases. They do have strengths, though. They are fast, and they can fight in hand to hand. And unlike other elite armies, they don't care as much about ranked infantry unless we start talking about 100 strong stubborn Skaven slave units.
So what do I see as their strengths?
1. Knights of the Realm. One of the best core choices in the game. Relatively cheap. Fast. Lots of high strength attacks on the charge. Heavily armored. Built in ward save. Can take magic banners. Damsels can high inside the unit safe from enemy attacks but still able to cast.
2. Wizards that can hide inside tough units and can cast Life buffs.
3. Trebuchets. A very cheap, and powerful stone thrower. Always, always take as many as you're allowed to take.
4. Pegasus Knights. Expensive, but they are the best fast cavalry in the game, and your primary means of silencing deadly Bolt Throwers and cannons. And they scare the crap out of people. They will draw a ton of fire power.
5. Peasant bowmen. These dudes are so cheap, and they aren't too bad. With defensive stakes they are very capable of keeping enemy skirmishers and fast cavalry away from your Trebuchets.
So what mistakes do Brettonian players commonly make?
1. They put a ton of points into their characters. No amount of points will make your Paladin or Lord able to match up with a Herald of Khorne. Don't waste points on trying to make him a badass. Stay humble. At the same time, no amount of magical items will make your casters into Teclis. Keep them naked, or with small equipment like Ironcurse Icons or dispell scrolls.
2. They buy any knights besides Knights of the Realm. Brettonians win based on key timing of their charges. Errants make it tough to do that. Don't take errants. Questing Knights are horrible. Grail Knights are insanely expensive. Grail Knights suffer from the same drawbacks as Brettonian fighty characters: people put a ton of points into them and they still can't get the job done. A fully tooled out unit of GK are badass, yes. But they will still get dropped by Chaos Knights of the the same points. Put it this way: make a mental list of the units that Knights of the Realm lose to, but Grail Knights beat. Not much. Not enough to make the points worth it.
3. Don't max Trebuchets. I can't stress this enough. You see people posting lists with one Treb and spending points on crazy stuff like Grail Reliquae. A strength 5 stone thrower is worth it's weight in gold. Take as many as possible. You do want to have a chance to beat Skaven, don't you?
So where would I start with a 2k Brettonian list? Glad you asked here goes.
-Lore of Life
[She will sit in a unit of KotR casting life buffs as best she can, and dispelling as best she can. If you can get Throne off and make it stick, Flesh to Stone knights are absolutely brutal.]
-Lore of Heavens
[She is here for the Lore of Heavens signature spell. Your opponent will have to try to dispell it, which means less dispell dice to throw at Throne of Vines.]
-Virtue of Noble Disdain
[This guy rolls with the Pegasus Knights, obviously. The Virtue is clutch to make sure you don't run away after you take the inevitable casualties from shooting.]
Knights of the Realm x 11
-Banner of Chalons
[Point these at Lotheran Sea Guard or Repeater Crossbowmen and break the news to your opponent that they can't stand and shoot. Adios, muchachos!]
Knights of the Realm x 11
-Standard of Discipline
[Generic beater unit.]
Knights of the Realm x12
-Banner of Eternal Flame
[Point and click kill a Hydra or Hell Pit or Treeman.]
Peasant Bowmen x18
Peasant Bowman x18
[Park these to block any access to your Trebs. Anything that gets in range gets shots. At worst, they buy your Trebs a turn or two of shooting extra. Best case scenario you effectively kill skirmishers and fast cav.]
Pegasus Knights x4
[I discussed their role earlier, but they are very clutch in attacking warmachines or shooty units early. They hit hard enough to attack small, elite units.]
Field Trebuchet x2
[I can't praise these enough. Str 5(10) stone throwers. They lower the boom.]
I don't think this is going to win you any big tournaments, but I bet it's a lot better than a battle-force list from a better book. It should at least give you a shot at pull out some wins. The key is multi-charging and staggering your charges. If you can't multi-charge, you charge with one lance and if you can't break the enemy with that single unit, charge next turn with another to bail them out. If you don't keep at least one of the knight units back as a tactical reserve you run the risk of getting your units tarpitted and that is bad news.