I am assuming you are not in a flat so do nopt have to get one that turns the gas off if you shut the lid; and that you are asking for gas because you do not have an electrical power point suitable for a cooker, or you prefer gas. You might however consider a dual-fuel cooker with a gas hob and a single fan oven, which can be run off a plug and socket (ovens use much less power than hobs). You can't run two electric ovens off a plug and socket. I have a dual-fuel and am very pleased with it. I agree a 60cm cooker is more roomy.
"Which" Best buys, freestanding cookers, gas, (best first). There were only three, and they all scored about the same:
Electrolux EKG6046KN £649 60cm wide. Upper oven has grill in it.
Zanussi ZCG561FW £249 50cm wide. One oven with separate grill compartment
Electrolux EKG6046WN £549 60cm wide. Upper oven has grill in it
(I include the more expensive ones in case you see one on special offer)
Apart from the best buys, the higher-scoring 60cm gas cookers under £450 were (best first):
New World 600TSIDLMW £428 (and similar in various colours)
Leisure GR6GVKP Gourmet Classic £427 (and similar in various colours)
Indesit K6G210G £305
Belling FSG60DO £375
You can read the full reviews at your local library