Costume Jewellery

These may help:

ARY D'PO • How To Store Your Jewelry To Prevent Tarnishing And Damage (arydpo.com)

How to store jewelry to prevent tarnishing and other damage
Even though some types of metal are more prone to tarnishing than others, most jewelry storage techniques will apply to any jewelry collection – whether you’ve got a large set of precious pieces, or you prefer casual costume jewelry. Here are some of the best tips for taking care of your collection.

Store your jewelry in a dry and dark place
Light, heat, and moisture are often the main culprits when it comes to jewelry tarnishing. You can’t just store your jewelry anywhere. It should be away from direct sunlight, which can cause fading. It should also be in a room that isn’t too hot and humid, such as the bathroom. The moisture in the air can cause tarnishing, especially on silver.

Keep your jewelry organized
It sounds simple, right? Unfortunately, it’s all too easy to take off your jewelry at the end of a long day and just throw it down on your vanity. Keep in mind that your pieces can easily scrape on each other and become damaged when they’re stored in a big tangled pile.

Avoid damage and tarnish to your necklaces by always doing up the clasp and hanging it on a dedicated hook. You can also use hooks to store your bracelets. In a similar vein, make sure you don’t just take off your earrings and throw them down anywhere. Put the backs back on and place them in a jewelry box or bag so they stay safe.

Use a compartmentalized jewelry box
A jewelry box that’s separated into compartments is probably the best way to store your jewelry to prevent tarnishing. At the end of the day, you can put each item in its own little compartment. This way, you can keep your soft amber bracelet away from your sharp diamond earrings, avoiding any unnecessary scrapes and dents. Be sure to choose a jewelry box that’s lined with a soft fabric, like felt or velvet.

Use silica or chalk to absorb moisture
Even if you store your jewelry in a location that’s fairly dry, there will probably still be some humidity in the air. You can add some things into your jewelry box to keep your silver from tarnishing. Silica gel packets are popular – and if you bought some shoes recently, you might already have some. Otherwise, chalk should do the trick too.

Keep silver in ziplock bags
If you live in a humid location, chalk or silica may not work as well as you’d hoped. If you’re having real issues with your silver tarnishing, you can also try storing it in small plastic bags, ideally, using the ones with anti-tarnish coating. Place each necklace, ring, bracelet, or pair of silver earrings in its own ziplock bag and make sure you push all the air out before sealing it up. This should keep it airtight and protected from moisture.

Keep jewelry boxes and bags
Most jewelry comes in a little felt box or drawstring bag. These can really come in handy when you’re traveling, because your jewelry box is probably a bit too big to take on holiday with you. Plus, taking all of your precious jewelry on holiday isn’t usually a good idea. Hang onto these little boxes and bags, because you never know when you’ll need them.

Also:
How to Keep Gold-Plated Jewelry from Tarnishing (byrdie.com)

Why Does Gold-Plated Jewelry Tarnish?
Invest in a Soft Cloth
Avoid Unnecessary Contact with Water
Be Mindful with Perfume
Store Gold-Plated Jewelry Strategically
How Long Should Gold-Plated Jewelry Last? (edited)

Avoid moisture, store it in a sealed bag or container, and keep it away from perfumes and lotions, Lastly give it a gentle clean with a soft cloth. With a little TLC

Don't wear it. Sadly there isn't really a better way - once your skin oils get on to the metal it will eventually degrade over time. That's why it's costume jewellery, it won't last forever without some blemishes. Even real silver will tarnish over time - solid gold is too brittle, so a base metal is usually gold plated, the plating will eventually wear off. Depending on the quality and thickness it can last a generation or two.

Thank you everyone for your replies, advice much appreciated

There's a Youtuber who does a lot of antique restoration videos (amateur ones) and seems to use Simichrome (metal polish) to great effect, though not sure what the equivalent is in the UK.