I agree, the past two summers have been very warm and dry in the SE, perfect for tomatoes if not for everything. But I started harvesting tomatoes this year in June and it’s only now just starting to slow down.
1. If you can sow seeds in February for a few plants, you will have to keep them in final pots indoors for a few weeks, but they can go outside in the sun on many days. That way, you are already flowering in April/early May even though your ‘greenhouse’ may be your living room for 4 weeks.
2. The fastest strains I”ve grown this year have been Maskotka and Red Alert. Red Alerts only germinated late May and they are now almost fully cropped. I kept 5 and didn’t pot them on beyond 15cm pots. I’ve got 1.25 – 1.5lb from each pot, which is 6 – 7.5lb at least of compost equivalent in a 30cm pot.
3. I don’t know where you are based, but Glacier is also reliable out of doors, even in cool climates. I sowed them in February and was harvesting from late June, with peak period July and the first week of August.
4. The March sowings are mostly coming to an end now, although the harvest was mid-August to mid September in the main: things like Alicante, Shirley and Ailsa Craig. I’ve kept seeds from my outdoor Shirleys and Alicantes each year, on the assumption that I am selecting for ability to grow in pots outdoors.
5. April sowings of Apero and Sungold have only really come into crop in September, although yields are pretty good on Apero.
I have tried the past two summers (but not this year) growing a plant in the soil – each time it was a Shirley. It did fine, but the harvests are later and the fruit sizes somewhat smaller, despite being in the sunniest spot of the vegetable patch.
Everyone’s situation is different, but my take on things is to accelerate the vegetative growth part of the cycle as much as possible so that, when the long days and the warmth arrive, you have all the photosynthetic machinery already in place to maximise fruit growth in July and August. I’ve also noticed that tomatoes really do respond to total sunlight hours, so if you have a spot where they will get 12hrs sunshine for 3 months (when there aren’t clouds) you’ll grow tomato plants much faster than if they get 6hrs a day.