Easy to see how the misunderstanding came about though, given that several posters on here have said thy think he's not BU.
It doesn't take much mental arithmetic to work out that the OP would be much worse off than her 'D'P doing it his way.
Say they both have a mortgage of £500 a month, bills of £500 a month and spend £200 a month each on food etc.
In two separate flats that's equal.
OP moves in with DP and is expected to pay her £500 mortgage, plus half of his mortgage, half his bills (there will still be some bills on her property too I expect, albeit smaller now) and if OP is cooking more does that mean she spends more on the food shopping now?
So now OP is spending
Her mortgage £500
Half his mortgage £250
Half his bills £250
Her smaller bills for utilities provided even if not used £50
Extra on food so now £250 plus time spent doing it.
Total £1300
Meanwhile DP now pays half his mortgage £250
Half his bills £250
And less on food £150
Total £650
Who is it that thinks this is a fair split for the OP to have the pleasure of her DP's company and half as much space to call her own?!