Burglar alarms - self install ones

[Houseplantmad]

I want to update our alarm system which is from ADT and also to part company with them as they are seeking mega £££ to upgrade it to wireless and also want to lock us into a contract, which I don't want.
A colleague has recommended the wifi alarm systems that you can install yourself but I don't really know where to start - can anyone share their experience please? We currently have a number of PIRs, two smoke/co2 alarms and two panic buttons. I don't need the system monitoring.

The Yale ones seem quite popular. i am not familiar with the latest ranges, but they still make a low-price self-install wireless (not wifi) range which is quite good if you connect it to your (landline) so it can dial out to keyholders.

I am not keen on wifi as you might have a power cut or internet failure.

@PigletJohn thank you, I will look at that.