When i did it i'm not sure i would have had enough money to do the M1 and then J1. I think i only just had enough money to convince the immigration people i could last on the J1, If i had spent some of my money before turning up in London i may not have got the J1.
If you can secure the J1 before turning up to do the M1 then that would be ideal. I was just glad to get the J1, the people in London took some persuading.