If you really want to go old school all you need are 2 mercury thermostats with SPDT switching. Use the cold level turn on outside and the warm level turn on inside. If you hook them in series they will only turn on your controller when it is colder than a certain level outside and warmer than a certain level inside. This will prevent situations where your house is already cold enough and a differential could still kick in and cool it more.
Actually it's kind of hard to justify analogue control systems at all any more. As soon as you need a mixture of logic, thresholds and time delays it's almost always cheaper and more relaible to use a micro. (I'm excluding "junk box" sourcing here). The nice thing about micros is that you can so easily adjust thresholds and have long time constants reliably at low cost. A (32 bit) micro with on chip ADC, timers etc etc will cost less than 50p in decent numbers - you can't get one reliable >5 minute time constant for that in analogue.
For the experimental one off the micro pays back because it's so easy to connect it to a PC or tablet for monitoring during development - and when you realise that actually you need an extra input or two and some more logic there's no trouble in adding it.
In about 1964 my father installed a central heating system with 100% mechanical "compensator" type controller - measured the outside temperature and adjusted the radiator water temperature accordingly - there was a copper bulb outside with a long flexible copper pipe to the controlelr/valve assembly near the boiler - elegant perhaps, with lot's of lovely machined metal parts, but not the way to do it now.
