Question about bugs / insects / gnats in soil. [UKGROWER]

Basically im using a soil bag which has been exposed to the outdoor rain accidentaly, no cash for other soil, ( actually compost ) but still.. well i took the compost out of the bag and broke it up with my hand and inspecting it with my naked eye for any insects or what not ( i know some are very small )..

my questions are .. >>

1 . Is their any product i can apply on top of the soil or during watering when i plant my seed ( haven't planted it, planning on saturday )

2 . What can i use in the grow tent itself WHICH IS EFFECTIVE to spot any insects

3 . Do spider mites / gnats even appear in UK climate? if so how ? climatE? conditions?

THANKS! if you can answer any.

simply..
- compost exterior bag got wet
- opened compost
- broke it up with hand, put in diff container
- checked compost ( naked eye, any bugs )
- made this thread asking!

EDIT : FORGOT TO ADD I LIVE IN UK, SO ANYTHING FROM POUNDLAND WOULD DO

anyone got an answer? or info

neem oil for most and make gnat traps with vinegar and soap

you can kill them all very very easy this is one of the best growing tips i have gotten and now i do it every grow, get a bag of white sand in any DIY store and put a good inch or two of sand at the top of your soil make sure to cover all soil, now after you water sand will turn hard and keep any bugs for getting out of the soil, it really is that easy! i now dont need or use neem oil anymore.

and yes gnats do appear in uk and they can be a real nightmare indoors with little flys everywhere!