Do you have any advice for moving into London (on a civil servants salary X-X)?
General moving to London advice:
As with pretty much everyone in London, you're going to need roommates. If you've got people already, perfect. If you don't, spare room is the usual choice. Studios are not viable choices for most people's budgets, and definitely not for a civil servant.
So after roommates, the next most important thing to figure out is where to live. Obviously I'm firmly north of the river but the south does exist too I guess (just please god not Clapham). But seriously, London is huge so your neighborhood will become your home base, especially in ~these trying times~ so do make sure you like it. Go and spend an afternoon wandering around potential neighborhoods, visiting shops and sitting in cafes and getting a sense of what living there is like. This map is really out of date but helpful for comparing prices between neighborhoods.
Moving with a serious budget advice:
The most important thing is try to live in a place in a neighborhood where you can have a fun London life and not feel broke all the time.
It's hard to get anything decent for less than £800/ month, which is likely to be a fair chunk of your budget. So because of that, make sure you live in a neighborhood that matches your budget and lifestyle. Even if you find an incredible deal on a flat share in Notting Hill, do you want to pay £6 for a latte? And only have expensive bars and restaurants within walking distance?
Also think about the commute to work and where you'll be going out -- I honestly focus more on the latter bc I'm lazy and won't travel long distances for fun, so better for me to be close to fun and further for work. If practical though, I find commuting on buses more pleasant (and notably cheaper) than the tube, so take that into account too.
Also! grocery stores near by. If there's only a waitrose in walking distance, you'll need to about for like 25% more for food shopping costs, so think about that too.
Once you've got housing sorted, you need to budget. I love Monzo's budgeting tools and couldn't have lived without them when I was first in London. Portion out your take home salary in a way that makes sense for how you live (so for me, that's lots of meals out, less on shopping) but definitely do try to save like at least ~100/month so you can go on holiday or buy a new bag or something.
Get to know the local, small, usually less expensive places around where you live so that when you do have the urge to go out, you have a reportoire of cheaper options. And if you're going on a big night out, definitely plan for at least some of the evening to be at someone's house -- it's really expensive to spend all night drinking in bars and then going out, better to cut out a bit of that by starting in and saving a big chunk of money.
Also also: can you cook? Maybe a good idea to invest in a few solid cookbooks (I'm a Salt Fat Acid Heat devotee) because if you do cook, even just your lunches and M-W dinner, London becomes much more affordable. That + little habit changes (like not getting a coffee in the morning, having a tea at the office instead) can make a big deal in terms of how flush you feel day to day.
Most people I know who had a bad time moving to London either moved into weird neighborhoods or spent too much of their budget on housing and weren't able to have any fun. Focus on finding somewhere that you can afford in a neighborhood that you like that fits your lifestyle.
Enjoy, you're going to love it!