To be fair to Thomas Higgins the reason they can offer cheap Letters Before Action is because they have a highly automated process in place, which does cut the costs considerably. It's also a bit of a loss leader because if the letter leads to a claim that's where there is a bit more margin to be made with the fixed fees.
As solicitors they (and we) obviously can't send out letters we know to be completely deficient with the protocol (especially the new Protocol which came into force last year for debts against individuals which requires us to send out about 10 pages of documentation with every letter!), but they are obviously reliant on the information that has been supplied to them by their client. In most cases for business debts it's sufficient to be able to state the name of the client (and any suitable reference), the amount owned, and if you're claiming interest or costs to include a schedule breaking this down. This is because most business debtors will know straight away what matter you're referring to, and if they are unsure they can inform you so they can be referred to the client. This satisfies the Protocol's requirements for sufficient information to be exchanged.
When it comes to issuing a Claim it can get trickier. Most debt recovery solicitors use a bulk interface directly into the court service computer systems. This was designed in the 1970s so it is very limited (for example you can only use 1000 characters on the Particulars of Claim and it can't cope with more than 2 defendants) which means it's inevitable that you won't be able to set out a detailed case within these limits. The Court Service is aware of this and takes the view that for straightforward B2B cases where money is owed it's sufficient to provide basic information in the Particulars (amount owed, the fact it's for goods/services unpaid, how any interest has been calculated and the first and last dates of any overdue invoices) within the system limits.
There is definitely a place for bespoke letters and Particulars, particularly for high value or complex matters, but where a client has a straightforward case it's entirely legitimate to use templated documents to speed up the process and keep costs down for everyone.