Soylent is available in the UK, although the pricing is… well, higher than in the US comparatively. Currently, they only sell through Amazon and only the RTD (both normal and Cafe) is available. Also, they do not ship outside the UK.
As per alternatives, there are many. That is one of the best things about Europe, that there are many brands (even if unfortunately not Soylent) that produce complete foods.
Let’s start with the most similar one to Soylent: Saturo (the RTDs are available in US, as well).
Saturo has similar ingredients and formula to Soylent. Macro split is 48C/18P/34F for the powder (available in vanilla and chocolate). Price is at €1.71/400kcal (without subscription. They also sell ready to drinks (similar to Soylent Drink) at €3/500kcal (35C/45F/20P).
Jimmyjoy (also available in the US). Cheaper shakes starting at €1.22/400kcal. they sell in bags of 5 meals (1 day) and have multiple flavours available (mango, strawberry, chocolate, vanilla, banana, neutral). Macro split 55C/15P/30F. They also have an active version with more protein.
Queal. They are the kings of sweet flavours in my opinion with great taster and starter deals. They have standard (€1.33/400kcal), light (€1.78/400kcal), sport (€1.48/400kcal) and vegan (€1.43/400kcal) versions. They also have bars. The vegan version is 48C/19P/33F. Overall they tend to have more carbs and protein than Soylent.
Huel. The british brand that it is also available in the US. Great brand with a very solid product (but you will have to buy 28 meals of 500kcal as minimum order. they have multiple flavours and price is €1.58/400kcal.
There are plenty more, including Jake (good brand), Pulve (similar to Huel), Mana (also soy based, similar to Soylent), YFood (for RTDs but not vegan), Feed (french brand with a huge array of products, Satislent (cheap shakes)…
Most brands should have discounts for referrals and starter kits. Perhaps that would be something that could work for you.