The ability of Passion flowers to cope with conditions far from their optimum is extraordinary. Many Passiflora that are usually only grown in a greenhouse will thrive & flower outside in UK from early summer to late autumn. There is no question that some plants are very difficult to flower outdoors in UK. For example P. holosericea which aborts peduncles before buds are produced. Others such as P. colinvauxii will flower well outdoors but the leaves start to look a bit green/yellow as light levels reduce. P. reflexiflora which flowers profusely over the summer is prone to virus problems & browning of leaves as light levels reduce. P. hahnii however, & many Tacsonia, will flower more readily outside as the temperature drops & day length reduces. I have a P. hahnii plant grown from a cutting from Ian Webb that has never flowered from him in a greenhouse, but is producing buds for me outside.
Most passion flowers, whether young seedlings or mature adult plants, will benefit from supplementary lighting if indoors over the winter months. You should plant outside any adult plant that has even a remote chance of surviving winter. Even the most unlikely of Passiflora are tough. e.g. P. 'Adularia' has survived min –6° C, 21° F. Meanwhile you should have taken cuttings of the plant you are testing outside earlier in the growing season. These can be kept indoors as small plants are easier to keep alive. My suggestions apply both to small scale seed germination & to keeping adult plants ticking over in UK winter.
Whatever your light source, an oscillating fan is essential. I cannot overemphasise this. It will avoid the plants overheating & will encourage sturdiness. Leave it on even when the light is off. Your plants will also benefit from this even while in the propagator & it will help avoid mildew problems. If plants are kept dry and watered sparingly from below most will come through fine with good air circulation and no supplementary lighting.
A fluorescent growlight which can be placed within 8'' is ideal as it will generate little heat. Always maximize light levels over winter, 12-16 or more hours a day will suit them. Specialised fluorescent grow tubes are very expensive, in UK £90. Even ordinary fluorescents which can be put within a few inches of the plants will do. Lights suitable for adult plants will also be OK but the heat many of them generate means that seedlings cannot be placed too near.
Supplementary lighting will also help some of your plants in pots that have been brought in to overwinter, and is essential if you want to flower them off season. If you have a spare room, in my case I have used my kitchen, you can't beat an oscillating fan & a 400-1000 watt halogen, mercury vapour or 500w mercury blend lamp. Blue light will encourage growth & lamps with more red will encourage flowering. Bear in mind though, in contrast to seedlings, that some plants will only flower with a short day length of 12 hours or less.