Carrera is a store brand for Halford's, which calls itself "the UK's leading retailer of automotive and cycling products." https://www.cyclerepublic.com/carrer...bike-2020.html

I'd imagine the frame uses 130-mm spacing but i cannot tell from the few descriptions I have seen.However .... since as far as I know nobody makes 126-mm frames any more, and nobody makes 126-mm wheels except in 27-inch for retro/C&V riders(I have some ion my '83 Cannondale) and since Carrera makes a variety of bikes .... I'd bet they produce frames with modern dropout spacing just for simplicty's sake. One article mentions that the bike uses 700c wheels ....

There are 8-speed clusters for freewheels--i think I got mine from Sunlite?--https://www.greggscycles.com/product/sunlite-8-speed-freewheel-230906-1.htm---but be aware----if you get an 8-speed cluster you will need to buy 8-speed shifters or you won't be able to access the added gear. And a pair of Claris shifters ...... about $100.

(Further, the bike comes with a14-28 cluster---if you need more speed, you are out of luck (smallest is 13-tooth) but if you need more climbing ability you can get a 14-34 freewheel pretty cheap. ($12--https://www.benscycle.com/shimano-tz500-7-speed-14-34t-freewheel/multi-speed_freewheel_shimano_25028_fw8142/product) If you jump to 8-speed you can get and 11-30 for $40 (https://www.benscycle.com/dimension-...fw7208/product) whihc would offer you the same 8-tooth low gear of you chose to run it with your current shifters and only use 7 gears (always an option.)

Considering the whole bike sells for about £300 ($375 US) paying $40 for the cluster and $100 for the shifters .... another $20 for the cables ..... I wouldn't do it. Brakes would set you back another $50 and then more for cables and suddenly you are spending half the purchase price to get a better bike-----sell the thing and buy a $600 bike that already comes with better parts, including a freehub and cassette (much stronger than a freewheel and cluster) and just better everything.

In the US we have a company called BikesDirect which sells no-name bikes with decent components for low bucks. Not sure if there is a UK equivalent .... ask around.

You have a 25-pound all-aluminum road bike with very basic components .... I wouldn't put any money into it except lights, a bottle cage, a seat bag to hold my repair tools, and a cargo rack. it is a very basic bike and isn't really worth the upgrades. (IMO)

On another hand, the bike should be serviceable for basic commuting and, if maintained, should last a good long time. it will have lower theft potential because it has little resale value---which is perfect for a commuter bike.

I would just clean it up, lube it up, tighten and straighten and adjust everything that needs it, and ride the beast for the next couple years. Save your cash for a better bike if you find you want one. Again, just my opinion.