Hello OP
My son is ten days old and despite being an extended breastfeeder of two I was forced by the hospital (not UK) to supplement him as his blood sugar was low.
I was scared this would cause nipple confusion/laziness/milk drying up - not coming in adequately especially as we were talking day 3.
Luckily none of these things happened. I was to give him breast then 10-30 mls of the ready made stuff they use in hospitals then more breast.
The HIPS brand came ready made up with one use disposable bottles - they said it could be used room temp so not to bother to warm up and could be used for up to 4 hours before throwing away.
The bottles are 90 ml so 3 extra supplements.
I have only ever done ebf with my other two (to 2 1/2) and am now ebf no. 3 (touch wood it continues) BUT I do have the ready made on standby if I need it...I will use it to supplement after bf if I need to.
I hate making bottles - am very lazy - and I like bf.
But mixed feeding is a compromise if nothing else you can see how much they are getting - mine was only taking 10ml ff before coming back to the breast but it was enough to get blood sugar from 38 to 60.
Also presume you have checked for tongue tie, allergies and if yours is actively sucking/latched - and that you are resting, getting extra 500 cals, vitamins etc
One last tip - breastfeeding tea - preferably with fenugreek - the latter promotes lactation.
My mum would swear by a pint of guinness once a week 
I am lucky I also have a toddler who helped my supply get going - has she not I would have forced my husband to suck and spit (do not think he would have swallowed somehow) -pumping has never worked for me personally.
Anyway here is a link to the ready sterilised/ready made stuff they use in hospitals - not promoting this as an exclusive solution but to cover you for weight gain whilst simultaneously bf
www.ocado.com/webshop/product/HiPP-Organic-First-Infant-Milk-Starter-Pack/73862011
it is six separate bottles of 90 with six presterile disposable bottles
so expensive but good as a short term solution - I think if you get powdered and sterilsing equipment and bottles etc you may end up giving a full feed, then baby sleeps/full, and your milk will dry up or you may be tempted to do ff full stop (nothing wrong in that of course but I got the impression you really really want to ebf but the weight gain is an issue so am trying to think of a short term solution for you)
available online tescos, mothercare,ocado
Lots of luck to you - let me know how you get on 
[tea] This is a link to the nursing tea I swear by (live in Germany so have a big choice here - do not know about uk so found a link with fenugreek in but maybe Hollands and barrett type shop have it)
www.immergruen.co.uk/Nursing-tea%281780852%29.htm