Both Blue and Red are important. And what's more, plants do care even about spectrum width inside each of the bands - They dont respond well to combination of just Red + just Blue LEDs, but they do once there is a combination of various Red LEDs + various Blue LEDs, all of different wavelengths, providing some sort of wider spectral distribution

Fluorescent provides good distribution in the Blue, unneeded (but otherwise harmless) light in Green, and enough Red/Yellow (for 2700K Fluorescents) but HPS is much better in making Red/Yellow

Both Blue and Red are important. And what's more, plants do care even about spectrum width inside each of the bands - They dont respond well to combination of just Red + just Blue LEDs, but they do once there is a combination of various Red LEDs + various Blue LEDs, all of different wavelengths, providing some sort of wider spectral distribution

Fluorescent provides good distribution in the Blue, unneeded (but otherwise harmless) light in Green, and enough Red/Yellow (for 2700K Fluorescents) but HPS is much better in making Red/Yellow

But it depends on the plants and what life stage they are in, HPS is better if your trying to get a plant to flower or produce fruit, but you want more blue if you want them to grow and produce leafs and roots, nature is interesting but warm and cool white tubes do work good, plants don't need a high CRI light to grow, they will grow under red and blue LED's without much issue or need for other lighting, but the philips SON-Agro 430 Watt tubes which are HPS with more blue spectrum do work pretty good for most applications..

And the guy at the hydroponics store I visited a few days ago is also growing plants under a set of red and blue T5 54 HO tubes just to see if it works, and they look horrible until you turn on the white lights and then they look great. 

And the guy at the hydroponics store I visited a few days ago is also growing plants under a set of red and blue T5 54 HO tubes just to see if it works, and they look horrible until you turn on the white lights and then they look great. 

I've thought about trying that myself (on something simple like lettuce & other salad greens) just to see how well it'd work.

I do have some houseplants with 4xF54T5 - a Red, Blue, 3000k, and 6400k.
I found the combination of Daylight + Gro-Lux WS seems to work quite nicely (wouldn't expect a whole lot out of just 2xF40 but as supplemental light, or for low-light plants its good)

For sure you can use them as grow lights but they are not very efficient. Daylight tubes used to be the main workhorse of lighting for plants until 1962 when Sylvania developed Gro-Lux, the world's first lamp optimised for plant growth.  That and its modern variants deliver a more valuable spectrum for stimulating photosynthesis reactions in plants.  If you would go back to daylight types now, you would need almost double the quantity of tubes to achieve the same rate of growth.

I agree w/ James, the Gro-Lux color seems to provide the most PAR (photosynethically active radiation), as it peaks mainly in blue and red, and very little energy is wasted in green and yellow. In my younger days I also experimented with growing plants under different light sources, the best fluorescent source I found was the Gro-lux, and the best HID was surprisingly the magenta/pink colored metal halide lamps used for architectural color floodlighting, which produced a very similar spectral distribution as the fluorescent gro-lux. I am actually quite surprised the horticulture industry never adopted the magenta metal halides like it did the daylight 6000K halides and HPS. I used the Plusrite MH400/U/MDX, the growth from my lettuce was phenomenal 

My experiences:

-Yes, Warm White will work, but will result in pale leaves.
-Yes, Daylight/Daylight Deluxe/anything /950 will work to some extent
-No, cool white will not, a plant will grow tall, leggy, and die.
-Grow lux is probably idea, obviously, being optimized for that...

All of these people wanting to grow plants indoors and under lights, it's a good job I'm not the suspicious kind... 

Somebody eventually needs to make a GroLux-WideSpectrum in F54T5
(I've seen something like GroLux-Narrow/Standard in F54 .. but like the F40 version its pretty dim (especially considering its a HO lamp - I would expect it to be intense to look directly at))

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Oh and I've found that the best lamp of all to add to a multi-lamp fixture for plants is an Actinic-03 (particularly if its something being started early inside (like tomatoes) that will be moved out once its warm enough)

Pot must not be legal in the UK either. There was a pretty big grow operation bust here a few years ago. I remember seeing it on the news, law enforcement hauling out dozens of fixtures out of the guys basement with 1000w metal halide lamps installed. They showed this bin in the driveway filled with maybe 50 or more spent EYE Iwasaki 1000w lamps. He had a 36KW natural gas or diesel generator in the backyard

Pot must not be legal in the UK either.

Nope, it's fast becoming a big problem over here.

He had a 36KW natural gas or diesel generator in the backyard

That's unusual, over here they usually just bypass the meter and steal the stuff.

Pot must not be legal in the UK either. There was a pretty big grow operation bust here a few years ago.

Even here in Colorado where its "legal", still not uncommon to see news stories where they've made another large bust like that.
People who grow that stuff are only allowed a limited number of plants (I don't remember what that number is, but am thinking 6 with max of 4 in bloom)...you can't have a whole basement full of it.