I assume you mean 30 miles each way, so over a year that's about 14,500. This kind of puts you on the cusp of diesel mileage, although it's not mega miles so petrol wouldn't be the wrong choice either.
That said, do you need to consider the ULEZ? If so, I wouldn't bother looking at diesels; you'd struggle to find one that's ULEZ-compliant for 6k.
For commuting I'd be prioritising refinement so here are my recommendations... assuming you want something medium-sized and 4/5 doors but if that's not what you're after I've got other suggestions.
Honda Civic. Both the petrol and diesel engines are very good, and it's a comfy place to sit on longer journeys. Very reliable, too.
Volvo S40. Small saloon, kind of based on the Ford Focus. Extremely comfortable inside, decent engine range. Volvo seats are terrific
Mazda 3. Mazda's engines have always been very good. The 3 is refined and nice to drive. Also reliable. Even the non-sporty ones look quite sort of swishy (personal opinion there!)