growing mustard plants to make mustard

There's no reason why you should not be able to get mustard growing and setting seed up in Manchester.  It's a fast growing hardy annual (grows, flowers, sets seeds and then dies) all in 1 year and can be sown directly into the soil. 

I would go to my local Indian or Pakistani grocers and get a big bag of mustard seeds if you want a cheap way of getting a lot of plants. You'll need a fair few to get enough plants to make it worthwhile 

You can grind the seeds and just add water to make a paste for basic mustard, or make grainy mustard.

250g mustard seeds
Approx 300ml white wine vinegar
2 tablespoons honey
1 tablespoon salt

Grind the seeds to your preferred texture and put them in a bowl with half the vinegar.  Leave 15 mins, then stir in the rest of the vinegar, honey and salt.  Cover and leave overnight and check if its OK or too dry - you may need more vinegar.  Spoon into sterilized jars and seal - keeps 12 months.

I've only made this with bought seeds so maybe try it first and see if that does the trick before you go to the trouble of growing all those plants 
 

I would try basic yellow mustard seed - the type you can get in seed packets for doing mustard and cress on the window ledge.

It would just be a lot more cost effective to get a big bag of seeds intended for cooking than mess around with loads of little seed packs 

Can you grow mustard in England? Colmans grow theirs in Norkolk.

The mustard used for green manure is white mustard and the seeds will produce mild mustards as Swing Swang says. To achieve hot "English" mustard you need the more pungent black mustard, I think it is a blend of both.

Black mustard is used in Asian cooking so it is worth looking in an Asian deli. Black mustard seeds are smaller and dark brown, white mustard seeds are larger and off white or yellowish.

The powdered mustard is ground and the husks removed. Whole grain would be easier.