We did a complete refurb, including an extension, a full loft conversion, new staircase in a different position, scaffolding, complete rewire, all new plumbing and heating, and new boiler and tank, new bathrooms, complete re plaster, new kitchen, walls moved, new flooring throughout, new doors throughout, new windows, decorated throughout and finally a nice new driveway once the skips had gone.
The whole lot came in at about £200k which included the architect fee of about £7k, planning permission and the builder's costs to manage all the trades on our behalf.
Your £100K is not unreasonable - depending on how much you are actually doing.
But the issue is not the cost, it's the fact you can't afford it and never could. You have spent money you didn't have in the first place.
I would recommend you speak to your designers and tell them you can't afford their twiddly bits and go back to basics.
I designed our kitchen. I used graph paper, measured absolutely everything. Scaled cut-outs of the various sizes of cupboards, hobs, ovens etc., and played around with it all until I was happy. Then we got Howdens to quote and refined it after the quote. From what you are saying you are just accepting everything a designer tells you that you need. That's their job - you don't have to agree!