Ignore the pathetic and vaguely racist comment on your tiny error in English – sadly many native English speakers remain incredibly narrow minded as they stamp around in their linguistic clogs. No doubt they find English the only language they need as they order their Stella in Torremolinos.
As for your day rate, I suspect from the range that you offer, you are indeed undercharging compared to market rates. As a general rule I would suggest that any consultancy company using you as an “associate” would be happy to make a 60% margin on your cost to them, i.e. for every £100 you charge, they should be happy to charge £160 or more. If they are getting £1000 a day for you, you should be able to get away with charging them about £600.
HOWEVER – it depends on your situation. If this is a stop gap until you find another permanent job, it may be better to stay in work for a lower than market rate than to risk being out of work and eating into your savings.