What is this white, winged insect?
At first I was thinking Trichoptera, but the antenna were not long enough and I noticed your size is quite small (~5mm).

The small size and body stance made me think of the order Neuroptera (the net-winged insects). 

Specifically, this specimen reminds me of the family Coniopterygidae (or the dustywings).


  A distinguishing feature is that like many other Neuroptera, dustywings carry their wings nearly side-by-side when at rest, whereas whiteflies carry them almost flat across the back  [the last part being related to the OP's original thought that their specimen was a whitefly]


Interestingly, unlike many other net-winged insects, the dustywings do not actually have "net-winged" venation -- just like the OP's specimen.

If this is in fact a dustywing, according to Wikipedia:


  These tiny insects can usually be determined to genus with a hand lens according to their wing venation, but to distinguish species, examination of the genitals by microscope is usually necessary.


Just as a reference, according to here and here, a similar looking species called Conwentzia pineticola lives in the UK. You can see a picture of this species below:



 Source: NatureSpot  

Comments on appearance and characteristics: 

From Catherine A. Tauber et al. (2009) in Encyclopedia of Insects (Second Edition):


  The Coniopterygidae is generally a very homogeneous family characterized by very small adults (forewing length 2–5 mm) with bodies covered by white waxy (“dusty”) secretions.


The family name means "dusty wing" [source] due to the usually conspicuous whitish dust of waxy scales on the wings of these insects. I can't tell if this "dusty" nature is apparent in your picture or whether your picture is simply blurry. 

Regarding the antenna, Tauber et al mention:


  Adults have chewing mouthparts, large compound eyes located laterally on the head, and multiarticulate antennae that are usually filiform (thread-like) or moniliform (with bead-like segments). 


You can see a similar looking (and similar sized) specimen from the U.S. here and one from Spain here. Additional images on bugguide.net seem to suggest that your specimen is in the subfamily Coniopteryginae.