Since you mention Delta, the options are rather limited for more legroom:

- Buy the business class fare
- Upgrade using miles from economy - BUT with Delta it needs to be on a near full fare coach ticket - probably in the range of $1500-2000
- Get an exit row. In advance, anyone can (unfortunately for us elites) book them on a 767-300 ER, which is used on the CVG-LGW flight. On the other Delta/Northwest US-UK flights, it's all 767-400s, 757-200s or A330s, all of which only limit exit rows to elites until you check-in. At check-in, if you're on one of these planes you can ask to be seated there if seats are still available.