You can find a lot of LC junk food and artificially sweetened syrups and things like that here.

http://www.lowcarbmegastore.com/

"Things to make sauces" that you mention, you really will be best off cooking from whole foods and from scratch. Stock up on pestos, tinned tomatoes or jarred passatas and double cream for your sauces, etc.

Xylitol is available at Holland and Barratt and all good health food/organic food stores, almond flour is readily available in the baking section in most supermarkets (for breading/frying/cake making), unsweetened cocoa powder also from your regular supermarket will mean that you can make some of your own, healthy treats.. sweetened with something less toxic for you and your family than Splenda.

Good luck!