Road Bike Lock Solution

Park next to a nicer bike.

One wise guy said that all bikes and locks should weigh 50 pounds to be safe. The lighter and more expensive the bike the heavier and more robust the locks.

Twofish megablock XL is a nicely designed bit of rubber and velcro which securely attaches any weight of lock to your top tube.

I use a Granit x plus mini 54 which doesn't come with a bracket, and the Abus brackets available for secondary sale are somewhat useless.

Found the twofish on Planet X site, thought it worth a fiver in a larger order of socks, and been pleased with it ever since!

Its light, it holds the lock securely, its easy to use, and easy to remove.

The 18" version of that looks not dissimilar to the hiplok z combo..  https://www.hiplok.com/product/z-lok-combo-black/

and while I was on their site, I noticed a d-lock with a cable which clips on to your clothing.. which sort of answers your original post - https://www.hiplok.com/product/hiplok-dxc-grey/ 

I like the hiplok site, have a root about, they have a lock finder on there as well. 

These are brilliant.

https://ottodesignworks.com/shop/ottolock

Perfect for the cafe stop and mine slips into a jersey pocket - don't even notice it.

I don't carry a lock as anything of any real security is too bulky. I would instead advise you to research cycle friendly cafe stops where you ride.

The cyclist pound is strong in this economy so most cafes that are serious about taking your cash will think about bike security and have somewhere to prop your bike away from potential 'lifters'.

To be honest my main concern is someone bumping me on the head while riding and legging with the bike.

I use a fairly thick cable lock that coils up on itself, and a good-quality brass padlock for coffee stops. The cable is about 2 meters long and will go through the frame and both wheels, and a bit of fencing or a bike stand.

A mini D-lock such as the Kryptonite Mini series or this one from Halfords might do the trick.

However, they're quite bulky and heavy. I only use a D-lock when leaving my bike in town for a period of time. For popping into a shop or stopping while out on rides I use a coiled cable combination lock. My LBS has the Giant Surelock Flex 15 - 180cm long x 15mm thick cable - for £20 (noticeably cheaper than similar ones from Abus & Kryptonite). It has a bracket to clip onto the bike. Something like that would surely be adequate.

Personally, for a cafe stop I just use one of these.  Fits easily in a jersey pocket and weighs nothing

https://www.cyclesurgery.com/p/hiplok-z-lok-armoured-reuseable-tie-P5234...

Thin combo lock (in seatpack) for coffee stops or stops at 'low risk' places (eg country pub).  For stops on rides where the bike's 'out of sight' and there's a higher risk of theft (eg in a town), I've devised a way of stashing a mini-D Lock by using a bigger seatpack (for the barrel) with the U part sandwiched between the seatpack and the saddle.  D Lock through a spoke, seat stay and onto the bike rack; thin combo lock through the front wheel, frame and onto bike rack.  Bit more weight, but better than losing the bike.

Something like this
https://www.amazon.co.uk/ABUS-Granit-X-Plus-540-Bracket/dp/B00G8QKAFU

You can't tell too much from the picture, but the Eazy mount is highly adaptable
The small bracket on the left can rotate slightly to vary the angle, then the bit it clips into can rotate around the seat tube and also when in position, it can rotate like a clock face ie if the initial position is 3 o'clock, you can make it 2 o'clock or 4 o'clock.

I always carry an Abus non resettable combination chain lock wrapped around my head tube for those quick stops. I don't trust those combination locks that allow you to reset the code as the barrels seem to be too loose so that they are easier to crack. The chain, I believe, is also a little harder to cut.

https://www.amazon.co.uk/ABUS-27164-Combination-Chain-Lock/dp/B00186V432

I do use the type of locks suggested by sergius to sucure my pannier bags to the rack on the occassions where I am doing quick stops whilst on tour and I do not want to have to unload and reload my bike each time.

It's probably worth me pointing out that I have security skewers on my wheels so locking up the wheels with a lock is not as much of a concern on a quick stop which enables me to feel a little more relaxed about the main lock that I use on a quick stop

I have a snowboard/ski lock that will fit in a jersey pocket.  It's not something you'd use for hours - but for ten minutes while popping into a shop it will do.

Edit: something like this:

https://www.evanscycles.com/abus-combiflex-2502-85cm-lock-EV310804?lsft=...