By the sound of it, Jake wanted to be home more - his first child is due the day of the 2013 Australian Grand Prix, and it's a lot easier to put a lot of time into being a dad if you're not spending a quarter of your life on a plane getting to and from racing venues. The BT football job is definitely not a passport to fame or fortune, even in the football-obsessed UK*, but it is based entirely in the UK.
The Radio Le Mans people are excellent, but their attitude to F1 is not very high. If they were invited to present, we'd probably get whatever the real topics of interest of the day obscured by rants about DRS, KERS, greenwash and safety car procedures. While there would be occasions where any of those would be appropriate, having them all done every race without fail wouldn't work too well.
Lee Mackenzie would be a good choice, I think. She did a good job of deputising for Jake for part of last year, is intelligent and can get decent answers out of people. She's not quite as good at making EJ make sense, but that's a skill one only really acquires on the job
* - The first reaction of many of my friends upon seeing this news was, "What? BT
has football matches?!?" If you want to become a famous football presenter, you need to be presenting the ITV or Sky matches, because relatively few people get BT compared to either of the other options, and indeed only started doing it recently after a monopoly investigation required some Premier League matches to be broadcast somewhere other than Sky.