Can't see the chapter and verse, but I understand that you could technically crop the land at the end of the term (which is 5 years not 10, unless combined with HLS) provided that you go through the Environmental Impact Assessment (EIA) procedure.
Under the present EIA rules, you would not have a problem, but in 5 or 10 years time who knows? The clever thing from their point of view is that although you have an ELS agreement probably indicating that there are no restrictions on cropping at the end of the agreement period, the EIA rules are completely separate.
Therefore there is nothing to stop the powers that be changing the EIA rules, either to prohibit intensive cropping or just making it extremely time consuming, complicated and costly to comply. Don't forget that in 5 - 10 years time there will be a whole new raft of environmental legislation relating to nitrates, phosphates, water quality, pesticide residues, etc. And you just trying getting anyone from English Nature or Defra to put an assurance to you in writing!
Nothing wrong with the (old) ELS scheme - but have your eyes open when going into it. I think Defra will find a lot of farmers choosing not to renew their ELS agreements when they reach the end of the first term - which will be a shame.