Bottom leaves yellowing, wondering if its any form of deficiency

First time grower here, I looked at other threads but couldn't tell from their pictures if there problem was the same as mine.
My camera isn't great but here are some pictures.

the final one are one of the first set of leaves that came through, I took it off the plant as it looked pretty dead to me.

Also curious how long people think before i should put them into flowering, thanks in advance for any help.

How often and much are you feeding them and with what.

haven't started on nutes yet as i read the soil i bought contained enough for 6-7 weeks, so at the moment it's just tap water that i've leave out for a few days. Only water them when i stick my finger in the soil a few inches down and it's dry.

What kind of soil? I would start to add the nutes now. Looks like a Potassium def. You got the watering down, just add the nutes now slowly( 1/2 strength ) then everything should work out. You should check your ph although I don't and have not had any problems.

plagron royalty mix is the soil, and have you go any suggestions on nutes, really not sure on what to get (living in UK)

Not sure what's available overthere. You want to look at the NPK ratios. Higher Nitrogen with lower potassium and phosphate for your grow nutes and for the bloom higher potassium and phosphate and low nitrogen. Hope that helps.

I really dont see any issues, in the pic they do look like they are due for a watering, but it sounds as if your watering correctly. I like to water mine every 3 or so days too. I always water untill I get a good amount of run off, you might want to add some larger drip trays under the pots so you can let some more water drip out the bottom. They are big enough to start useing nutes now, if you stay with organic fertilizers you can feed them almost every watering and not worry about burning them up. I like to use Fox Farms products like Grow big, Big Bloom and Tiger Bloom. The yellow leaves that came off are completly normal, you will see some of the older fan leave yellow and fall off as your plant gets further into flowering, some strains do this more than others. You can start flowering whenever you want, just remember most plants will double or triple in size during flowering so make sure you have enough room if your not going to LST or suppercrop.

Stlmatt

I think Plagron mix is enough feed for 2~3 weeks, Keep an eye on your PH(water) and when you are ready to flip remember they will double in height during flowering....or even triple if a Sativa strain.