As the United Kingdom leaves the European Union, there is debate as to whether the UK should remain in the EU Customs Union (or in a bilateral customs union with the EU, which I believe is almost the same in practice), or leave so that it can negotiate its own trade treaties. I can understand the arguments in favour of the UK leaving the EU Common Market if it does not want to guarantee the EU Four Freedoms, and I can see some people dislike the European Court of Justice having power to enforce EU law in the UK. But I don't understand the customs union bit. Presumably, a larger block like the EU should be able to negotiate better terms with other parties than a single country like the UK; both because it has more leverage, and because it has more access to experienced and skilled negotiators. But what could be the advantages for the UK to negotiate its own trade treaties, outside of the EU Customs Union?
The problem is that the EU has to satisfy demands from all member states. In particular, the UK isn't as protective as a few other member states. The EU may have been setting unreasonable tariffs to protect some non-UK industry, and in exchange got a worse deal in return. The UK on its own can make a deal that only needs to take into account British industry.
A similar situation exists in reverse. The UK only needs to negotiate access to other markets for goods produced in the UK; UK exports are only a subset of EU exports.
As an example, the EU as a whole exports a wide variety of foods. Food is an important discussion point on trade treaties, as many countries want to protect domestic agriculture. The EU may need to make concessions elsewhere in order to placate EU farmers. But the UK exports almost no food. In fact, the new British trade treaties may open up the British market for food imports, leaving EU farmers to compete with farmers from around the world on the British market. This might lead to lower food prices in British supermarkets.
Any time one is part of a group, then the goals and outcomes of any individual are subject to consensus and compromise even before a negotiation starts.
An individual negotiating will always have their own priorities exclusively paramount from their side of the table.
The trade-off is that, while I might only have my considerations from my side of the negotiation, I'd lack the leverage that I might have, as part of a larger group, to extract concessions for my considerations
The perception is that by negotiating trade deals individually, the deal can be tailored to the individual country.
Another advantage for very large economies is that they have a very strong negotiating position compared to smaller ones. Post-Brexit, the UK will be desperate to do deals quickly to make up for the loss of access to the EEA, so countries like China and the US are in very strong positions to force the UK to accept terms because of the huge imbalance. Failure to do a deal will hurt the smaller economy far more than the larger one, if the larger one even feels it.
Another example is the US withdrawal from TPP. They are now trying to do individual deals with countries like Japan, and Japan in particular is trying to resist any deal and stall for time because it doesn't want to accept the terms that the US is trying to dictate. When it was a large group of countries negotiating it was difficult for the US to make demands like lowering beef tariffs, but when it is just the US and Japan and the US can use other areas of trade as bargaining chips, it becomes much harder for Japan to resist.