Smmb,
We did exactly as you intend doing. We had a chimney breast protruding into our kitchen and completely removed it and extended the kitchen too. We didn't apply nor needed to apply for planning permission either - extension under 20sqm. Obviously an RSJ was needed to be put in place of the chimney breast and the upstairs chimney breast in the spare bedroom needed to be adequately supported during the process. Other than that there was no issues. Best of luck with it.