There's nothing pretty about this advice I'm afraid.
In fact that is the first piece of advice. Nothing pretty. These people rarely have any appreciation of the maintenance standards or the replacement parts, unless they're blingy. And yes, a Brooks saddle counts as blingy. They primarily clock what it looks like, so the better it looks, the more likely it is they will want to streal it.
They won't know they're quite good rims with new bearings set to perfection. Upgraded bottom bracket? they're none the wiser. Just don't wash it. Well, don't polish it anyway. You could even crusty it up a bit, but don't overdo it. A bit of gaffer tape. No more than 6 inches. And a sticker that has nothing to do with bicycles. Perhaps a couple of cable ties. (The releasable ones are actually quite useful sometimes.) If this all seems too much then at least make sure it's not the prettiest bike wherever you park it.
If this is all disappointing because you want your bike to look flashy, you're not going to like this either. Get rid of the quick release wheel spindles. They are pointless. You do not have a support vehicle with backup wheels ready to get you back on your way to the finish line. Even if they just mess with them you could lose your front teeth. If they get taken or damaged you may leave the remains where they are and they can come back.
Possibly the single best security device you can employ are things called Pitlocks. The Tea Leaves know what they are and know they're a faff. They can get them off with the right tools and a bit of perseverence, but it takes time, which is good, and it leaves them needing a replacement option. They're better off looking elsewhere. And seeing as you've got the little nut spanner thing on your keyring, you always have a way of getting your wheels off.
The second great benefit of using Pitlocks is that you won't need to carry around an extra cable to lock the wheels and you won't need to carry the seat around with you when you're off the bike. The added convenience of this arrangment is a pure joy. You'll wonder why it was ever different. You'll also not do that thing where you lock someone elses bike to yours because you're in a hurry. By the way, I don't sell Pitlocks, but they really are that good.
The next thing to consider is the actual lock. Anything is better than nothing but if you're going shopping there are a few things to consider.
You will probably find a D-lock with a frame clip to be the most convenient type of lock to use day to day. More money, more better is often the way but try and get one which locks both ends of the 'U'. Either with ball bearings or rollers. This makes them harder to cut off as it usually requires two cuts to remove them but make sure it doesn't rattle about too much when it's locked. Both ends being locked tends to make them stouter in defence of a prying type attack as well as they cant pop the unlocked end out of the lock body. I've had a Abus Granit for nearly 15 years and it's still functioning perfectly. Long enough for me to have completely forgotten what it originally cost me.
I think this is about the best you can do for a transport bicycle. Locking it to something is obviously crucial but the way things are going it's quite common to find a bicycle rack fairly nearby. Certainly in Hackney, but please lock yours to the road side of the rack if it's free. Blocks less of the pavement and fewer people will moan to the council. If you live in one of the Evil boroughs up west I guess it serves you right, but you can always cycle to other boroughs and lock it up there while you go about your economically stimulating business.
Any kind of chain you'd be prepared to lug around on a pushbike will not provide any measure of security if it's even momentarily out of sight. Most of the commercially available chains are sourced offshore and bought by the tonne. These are then cut into lengths, coated with added marketing and given a ruddy great price tag. Even if they look physically impressive they rarely are from a metallurgy point of view and can usually be cut with bolt croppers.
The only chains that are truly resistant to bolt croppers are made by Almax and Pragmasis. However, they seem to be made from pure gravity, so they are quite heavy. Probably not suitable for carting around with you but maybe useful if you have a regular locking spot. They are not cheap but they are worth the money you pay and just their prescence is a deterrent.