misting a bit will do.but from my knowledge,Highland neps dont really need high humidity.whats important is the cool temps.not cold but cool!15c-23c will do.they're also can withstand winter.i bet at UK,its easy to grow highland neps unlike my place.sun also needed but filtered sun.
but a little misting a day will just do the job for safety.
also try to get some of those N.Hamata!they look cool to me.
Hi Growing nep's inside is tricky, you will still need the same amount of light as outside growing and while they will grow in lower humidity is not recommended,as it affects there growth , eg: tendril deformities , low pitcher rates , smaller pitchers , pitchers deformities ,and over all growing problems !!,and the same with the light factor as well,its most important. temps ,warm days and cool nights , very important or you are going to have problems , J
Yeah John is right! The Neps wants their natural conditions and them require them. So it is neccessary to give them. Is the humidity isn't enought they don't produce pitchers. Is the light isn't enought the leaves are smaller and etc. and etc.