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If I would like to heat my house in the winter with solar thermal power, there is a problem like you said. Little sunlight in the winter.
However, most solar thermal installations are rather small. 1-2 m^2.
What about just installing a thermal unit twice its size? The cost of the unit itself (that part at least) would be about 35% of the total, so I could double the capacity for little money.
The amount of solar energy falling on 1m² of optimally inclined surface in the UK is from 1200 to 900 kWhr/year depending where you are. There is a useful map here. A solar thermal system will convert 40% to 50% of that to useful heat in your system. Say 45% of 1100kWhr equals about 500kWhr/m².year
However only 23% of that solar energy will fall in the winter six months. That is 115kWhr/m² of output spread over 6 months or 0.6kWhr/m² per day on average. This ignores the fact that vacuum tube systems are somewhat less efficient in cold weather and flat plate systems very much less efficient. That is bad enough but averaged over a shorter period it is much worse. I have both a solar thermal system (2.7m² aperture area) and a 17m² photovoltaic array (and a 10.8kW heat pump) Although I have fitted flow gauges and thermocouples to the solar thermal system I have not yet connected them up. However the relative response of the two systems over time will fairly similar and the photovoltaic system is instrumented. I average about 46kWhr /week of electrical generation but in the first of June this year I generated 100kWhr while in the first week of January this year I generated 4kWhr all week. A solar thermal system would be expected to have the same sort of ratios.
Thus the 500kWhr/m² per year may be an average of 9.6kWhr/m² per week but you must expect 21kWhr/m² in the best week of summer and only 0.83kWhr/m² to last the whole of the worst week of the winter. It depends on your house but you would be lucky to get through the worst week of winter with less than 200kWhr of heating (1.2kW continuously to heat the whole house and 500kWhr would not keep many less well insulated houses warm for a week in the depths of winter.
You would thus require 12m² of aperture area solar thermal system to make a 5% contribution to your 200kWhr week's heating. (or a 2% contribution to a 500kWhr heating requirement).
Even if you did invest in such a system and you had enough room for it (with a vacuum system this about 24m² gross area) you would find that in summer you would have weeks that gave you 250kWhr and that is more than you are likely to need for domestic hot water. You would have to spill some of that heat in a radiator or some such, wasting the output of your fairly expensive system.
Solar thermal systems in this country are a good way of providing almost all of your domestic hot water in the summer with useful contributions to some other form of heating in spring and autumn and a token contribution in winter. My 2.7m² system does just that and I installed myself for about £1900. Those that sell solar thermal system for central heating in this country (and I have come across those that do) are plain dishonest.