It is at least a 4-hour job, maybe more, just to remove the over-sill, clean back for welding, trial fit the new sill, bend and adjust as necessary, weld the new sill on, grind back, linish, prime and paint. At £40 per hour that is £160 + VAT. So £200 seems a bit cheap to me, unless it's going to be a bodge job.
That is without the additional work which ALWAYS becomes necessary when over-sills have been fitted. For example, new jacking points will almost certainly be needed together with a repair to the end of the cross member where the jacking point fits. Then there is the probable damage to the floor where the over-sill was welded and corrosion to the inner sill.
I don't think I've ever seen an over-sill where there was not significant corrosion underneath it. In fact, that is why those cretins fit over-sills in the first place, to hide the crap underneath.
Sorry if this seems negative, but it is better to be prepared for the worst.
The problem can be that when the over-sills are cut off, you will get a call saying that your £200 job has just gone to £400 as a result of all the additional work. Then there is no way forward or back without simply paying what is required to get the car safe and complete.
How many times have we all seen that? Too many!