goldcrests - pishing works on goldcrests so worth a go (i posted on pishing earlier in the thread)
with old world warblers it's extremely useful to nail it down to family first if unsure of ID, and the families you are gonna run into in the uk are:
Sylvia warblers - whitethroat, lesser whitethroat, dartford warbler, blackcap, garden warbler etc
Phylloscopus warblers (birders term 'phyllosc') - also known as leaf warblers, willow warblers and chiffchaff etc
Acrocephalus warblers (birder term 'acro') - largely reed and swamp birds, reed and sedge warblers etc
Locustella warblers - typically live in thick vegetation, grasshopper and savi's warblers (reed beds and rare)
Hippolais warblers (birders term 'hippo') - all these are rare to uk but some annual rarities (eg melodious and icterine warblers), so i wouldnt worry about them
useful general reading for a quick overview
https://www.bto.org/understanding-birds/birdfacts/bird-families/warblers and you can click on the individual bird species (there are tons of rare and some extremely rare birds in there so i'd stick to looking at the common stuff)
from what you said if it does come down to whitethroat/lesser whitethroat, simplest clue even if a juvenile bird (not as distinctive) - if it's a whitethroat there will be some rufous tones in the wings, if it's a lesser whitethroat there wont be - calls are very different too (tho birds can turn up in unlikely places for its species, generally, the habitat and any calls are vital parts in helping IDing birds, so apart from what it looks like, always be mindful of everything else it's telling you)
many (there are always exceptions) warblers can be seperated into their family by how long the undertail coverts* are in relation to the tail, once known and practiced, this approach is very useful in quickly eliminating species and narrowing down your options with an unfamiliar bird - i was looking for a pic that illustrates this but can't find one online, if i ever come across one i'll post it and refer back to this post
i did start detailing how this works with the warbler families and looking for images of individual birds to illustrate the differences but it got so fucking long winded i'd take half a fucking thread lol - so annoying cos if i was stood next to you i could explain it in a minute
*
ah fuck it, just to illustrate very quickly two families to show you what i mean
two pretty plain birds that superficially look very similar and both are in reed beds - let's assume you cant hear the songs cos of traffic or distance etc and there's nothing else near it to give you an exact size, now you can look at head patterns, the tones of the plumage and other subtle clues trying to work out what you're looking at......... or
you can look at the undertail coverts and notice that the top image they are long, yes, but fall just over halfway down the tail
and in the second image you can see they are very long and almost reach the tail tip
the top image undertail coverts tell you immediately that you're looking at an acro, and the second image undertail coverts tells you immediately that you're looking at a locustella
so simply looking at the undertail coverts you've taken a huge shortcut and stopped tons of fucking around with much more difficult elements to assess
so let's say it's the bird in the second image you are trying to identify, you've gone fuck it, what's that, looks a little big but can't be sure, oh look at those undertail coverts, fuck me, it's a locustella - and the fact it's a plain bird singing in a reed bed gives you only one option, hold the fucking phones lads, i've just lucked in on a Savi's Warbler! woohoo im big billy bollocks for the day and the rest of the guys will be buying me drinks
