I know those figures because i used to build them for a living working in an architectual practice, have built LOADS of housing projects and a few private 1 off houses. The most impressive was this 5 beds (4 with ensuite), Study, with reception rooms, diner/kitchen, living rooms, extra bathroom, utility, Georgian windows, detach house and all come under budget for £200k. The couple already have the land, so i dunno what that costs them in the first place.
Buying land without planning is a risk, you can of course apply planning for land that doesn't belong to you but you have to notify the owner you are doing so, so if you get it, his selling price will sky rocket. You can of course enter into a contract to him that you will buy the land subject to planning approval. The other way, a slightly cheaper way is to apply for outline planning approval, its basically planning to build a house on the land, very few drawings thus cheaper. Once you get it, you can apply for full planning approval.
BTW - £1,200 per sq meter is per floor, double that for 2 storeys, that said, the garage would be cheaper to build as the finish is not the same as the main house.