Beet costs £500 an acre to grow (including a reasonable rent). Beet for 2009 after delivery is £25 a ton at todays exchange rate. So 20t/ac is your break even excluding SFP. In my opinion, you therefore need 28t/ac to make it as profitable as growing rape. There are obvious pluses to beet ie no weeds left, and quick cashflow. Downers are chance of a messy harvest and therefore a terrible wheat crop after.
You will have a much better wheat entry with peas. Is the contract with Frontier for Kabuki? If so, then dont budget for 2t/ac. IMHO unless you are a risk taker, very experienced, or on some kind land then avoid marrowfats. The chances of not having a harvest vastly exceed beet. Neighbours struggled to fill a 14t trailer from 30ac. Oh, I havent seen the contract, but look carefully at it. I have never seen one that doesnt royally shaft you for waste, stain etc which will really eat away at the price. If you are on nice pea land, you probably ought be growing beet in the rotation too.
Look at the whole farm rotation and profitability, and grow what you know you can do. Dont jump into marrowfats thinking they will be anything but a nightmare to harvest. BS may be a monopoly, but there are only 3 pea buyers, and you can bet your bottom dollar that they are all best mates waiting to consign your peas to the "feed" pile, whatever they are like.