Is it too late too transplant

[sage]

I have three sativa/indica plants in 16inch(40cm) diameter pots with a depth of 12inchs(30cm). The total volume is roughly 13litres or 2.8 gallons. I had always planned to plant in the ground but was advised not too. Now i'm worried i'm not gona get near the yield I would if I had put in the ground. Do you think I should cut the bottom of the pots off and bury the pots in the ground? I think the pots are too big to directly transplant.

I germinated the seeds on the 25th april and put them into pots in my polytunnel around the 2nd june. One plant is about 20inchs tall, the second is about 15 inchs. The third has had very stunted growth(8-10inchs) and its leaves are very green and shrivelled.

I'd really appreciate any advice you seasoned growers could give me. Thanls
P.S. I live in the UK if that matters

[Hick]

It isn't "too late" to cut the bottoms out and let those roots spread out, but I'm uncertain as to the ability for them to mature to harvest with weather conditions being what they are in the UK.