After basic you'll do Phase 2 which is ACS course VC30 or whatever it may be. That is either 2 weeks at Larkhill or done in house with your Regiment either as part of 2 week ACT or modular over weekends. That's normally how it works to get you traded, there used to be a £1000 bonus for completing this.
From that you'll then go light gun (are 266 still light gun?) again two weeks or do your next ACS course if going CP or another course based on going FST or Arty Log.
Downside I found with progression in Artillery is course durations as they tend to become lengthy with some heading into 5-6 weeks!