P700s are the earlier of the two, dating back to the 1950s. They're distinguishable by the round emblem with LUCAS on it in the center of the tripod. I don't have my catalogues handy, but ISTR the P700 took a typical pre-focus headlamp bulb common to most other Lucas 700 series (home market) headlamps.
The PL700 is later, pretty much 1960s vintage. They're easily spotted by the somewhat triangular "shield" in the center. Again, ISTR the ones I had took a "Type A" headlamp bulb, which had a three-prong plug built into the base of the bulb.
Originally, both took tungsten bulbs, so they can't begin to compare with any of today's halogen (or better) bulbs. But in both cases (and I put many miles on a TR3A with P700s and a Spitfire Mk3 with PL700s), lighting was substantially better than with the readily available tungsten-filament sealed beams of the day. The light pattern was not quite as distinct as, say, a good Cibie* of the period, but it was much better than a typical GE, Tung-Sol or whatever sealed beam from Western Auto or your local Esso station!
*As much as I loved the Lucas lamps, I did have a set of Cibie "Optique" headlamps (I think that was the name) that took the same Type A bulb as the PL700, but with the typically wonderful Cibie lens that beautifully focused the low beam pattern out directly in front with a low, sharp cutoff, and a nice diagonal sweep up and off to the RH side of the road:
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(Above: really bad ASCII depiction of light pattern!)