Heathrow is with the A319 except Saturday evenings and Sunday mornings when it is an ERJ 145. The Birmingham/Cardiff flights are probably with the 733.
On the Birmingham route, I agree that there will be excess capacity this summer. Since 2004, with the introduction of Thomsonfly Coventry flights, Flybe passenger numbers have decreased considerably and bmibaby can only exacerbate this.
Nevertheless, the Flybe fares between Birmingham and Jersey are notoriously high thus competition is welcome if it drives down fares and generates new passengers. But with the likelihood of flights going 2x daily, as the bmibaby press release highlights, then something will have to give.
Definitely a make or break summer for Jersey Airport on a number of routes. It will be interesting next year to see what changes there will be as a result.