Swindon is much closer to Avebury so will be better for trains. Also First Great Western still run guards carriages on most trains to London which is a far better option for bikes. The trains are more frequent and, I think, a little faster. You can leave at 5.23 and be in Paddington by 6.24. Plenty of time to ride across London to Waterloo before the Salisbury train arrives.
Swindon is much closer to Avebury so will be better for trains. Also First Great Western still run guards carriages on most trains to London which is a far better option for bikes. The trains are more frequent and, I think, a little faster. You can leave at 5.23 and be in Paddington by 6.24. Plenty of time to ride across London to Waterloo before the Salisbury train arrives.