Samlack99

In America, silver has no VAT and thus has much lower spreads than it does here, so it is much easier to stack silver efficiently over there.  You will see a lot of silver stacking YT channels in the US, but the tax conditions and market are very different here, so you should take American stacking channels with a pinch of salt.
Silver is quite hard to stack efficiently in the UK.  VAT really ganks the dealer market, so you can expect to lose about 30% or so of the value if you buy and sell back to dealers.  The only really cost effective way to stack silver is to buy and sell on the secondary market, where you can get much smaller spreads.  As a starting point for this, I suggest you reach out to @theman73 and see what he's got available at the moment.  I've bought plenty of stuff from him in the past; he's a good egg.  Also, keep an eye on the trading forums and get a sense of what things sell for.
Gold has much lower spreads than silver as it doesn't attract VAT.  The entry point for gold is really half sovereigns, although 1/10oz coins often turn up at decent prices on the secondary market (such as here on TSF).  Sovereigns are really the sweet spot for stacking gold here in the UK, as they have a killer combination of low premiums, small size and liquidity and exemption from capital gains tax (they are still technically legal tender and exempt because of this).  Sovereigns aren't the be-all and end-all of stacking gold in the UK, but they are a good starting point.  If you want to buy some sovereigns, I suggest you get in touch with @Belmont, @CallumHSLimited or @shawy2510 as a starting point.  There are other vendors flogging sovs here, so keep an eye on the listings and see what comes up.
If you're going to get into sovereigns, it pays to have a bit of understanding about the numismatics of the coin, for example being able to do a basic assessment of the grade, or knowing which designs or mintages might attract a premium.  Shields are a good example of this as they tend to sell for about 10-15% over spot, where 5% is more normal for sovs.  Get hold of Marsh's The Gold Sovereign, Allen's Grading British Coins and the current edition of Spink's Coins of England and the United Kingdom as a starting point.
Sovs are not the be-all and end-all of stacking gold in the UK; it would also pay to get an idea of the typical premiums associated with other items such as 1oz coins, 1/4oz coins or bars (take a look at Tavex's house brand 1g bars as another entry point to dip your toe into the water with gold).  Look at the prices on the dealer web sites, then take a look at the sales postings here to see what the coins trade for on the secondary market.
Note that the price of gold is quite high at the moment.  I can't give you any advice as to whether it will go up or down in the future, but look into cost averaging as a strategy.  Silver might well go up, and a lot of folks think it will, but I can't give you any prognosis on this either.  However, the huge spreads on the main dealer market mean it would have to really go up a lot in order for you not to lose on it, so you really need to get ofay with the secondary market if you want to do silver.