Question A would be, how much do you want to spend?
I assume you are stealthing also given that you're in the UK. Here is a short list of some typical small mics and their price, with comments... Also, with most of these, you'll need some kind of power supply. I recommend a simple small battery box, either the ones made by Chris Church or the ones made by Sound Professionals. They are $65 or so. People will try to push you to get a preamp; I think that is a waste quite frankly for stealth rock recording with inexpensive mics, and it costs more and takes up more space. Frankly I can't tell much or any difference between my DPA 4021s running into a preamp vs. running off a basic 48v power supply direct into my Sony M10, for stealthing anyway.
The "Big 4" standard inexpensive mics:
Church CA-11 (under $150, see "Retail Space" below). Pretty standard, solid mic. Handmade, will take some time to get to you.
Church CA-14 (at or under $150). Larger mic made by same manufacturer. For very stealthy situations, maybe not recommended due to larger size.
SP-CMC-8 (~$250). aka the AT943. Comparable in quality to the Church mics; mostly a matter of taste. Bass is pretty rolled off as it is with the CA-11
SP-CMC-4 (~$200), aka the AT U853. Larger than SP-CMC-8 or CA-11 mic, will need to be worn in a hat. Better bass response. Comparable to CA-14.
All of the above come in cardiod and omnidirectional; if you can swing it, I recommend both; if not, get a cardiod mic. The Sound Professionals (SP) mics also come in hypercardiod. I don't recommend buying that unless everything you do is in a stadium.
Mid-range priced mics:
Countryman B3 - gets raves around here from those who use them. About $330 I think. Extremely small. Omnidirectional only.
Audix 1280c + Church active cables - Around $400. Body and cable is the same as CA-11 cosmetically, but you buy an Audix capsule that, IMHO, has a bit better performance than the CA-11.
AKG ck9x active setup. Takes a bit of doing but IMHO sounds better than all of the above. You buy two AKG mic capsules then have someone wire them up and make a custom battbox. Church can do a preamp for these. For someone starting back out, probably not a good option, but it's cheap (~$500) and sounds awesome.
Nevaton MCE400. Omnidirectional only. Only has XLR connectors, which is less than ideal, but sounds great. $400 used right now in the yard sale.
Sonic Studios DSM-6S. The mic of choice in the 90s, it has had a harder time competing on price as these are about $600. They are, however, excellent mics and the recordings made with them attest to that.
High-end mics:
Probably just an aspirational list for you at this point, but:
DPA 4061/4060 - tiny like the Countryman mics, very stealthy. Omnidirectional only. $1000
Schoeps Mk4/mk41>CMR cables or KC5 cables>NBox or TinyBox - $3000. Amazing sound.
DPA 4021 - amazing sound, and unlike the Schoeops, does not require any special cable (hard wired with its own cable, bodies are small as-is). $3000
The standard advice around here these days is, "buy Church Audio mics." And it's not bad advice. However, a) there are lots of mics out there and b) a lot of the people recommending solutions haven't tried very many of them. I don't know everything or even as much as many people here. But I have tried lots of mics and taped a fairly big number of shows... The bottom line is, it's a personal preference and a money issue. Generally speaking, sadly, you get what you pay for. Spend more, get better mics. Buy nice or buy twice, etc. (advice I give but failed utterly to take). All of that said, the Church mics are hard to argue with given that a) they work, b) most people like how they sound, c) Chris is a nice guy and very supportive of TS.com and d) the price is right, so if you don't like them, you haven't sunk a fortune into them.
Hope that helps.