I’m a H&S officer at a construction firm and builders merchant one of products we produce are concrete lego blocks approx. 1.8m x 0.6m in size the type you would see in a recycling plant
that form walls to separate waste, a customer wants to place an order for some blocks and has requested that the blocks hold a A1fire-resistant classification in compliance with REI 240 standards and are effective for at least 4 hours.

Does anybody know such a firm in the UK who would test and provide this certification ? 

I’ve tried 2 companies i found on the net but they seem rather slow to respond and time is the enemy.

Decision 94/611/EC 4th October 1996 Materials considered as reaction to fire Classes A

Products should be made only from one or more of the following materials if they are to be considered as Classes A without testing. None of the materials in the table is allowed to contain more than 1,0 % by weight or volume (whichever is the lower) of homogeneously distributed organic material.

Concrete - includes ready-mixed concrete and pre-cast reinforced and prestressed products

Aggregate concrete (dense and lightweight mineral aggregates, excluding integral thermal insulation) - May contain admixtures and additions (e.g. PFA), pigments and other materials. Includes precast units

If you want an actual Reaction to Fire test then you need a Notified Body - UK list

http://ec.europa.eu/growth/tools-databases/nando/index.cfm?fuseaction=country.notifiedbody&cou_id=826

But these are all very busy at the moment thanks to last year - you could try one of the European NB's

Do note that dependent upon the deal or no deal come March 2019 the status of Notified Bodies in a particular country for conducting EN testing may change

https://www.gov.uk/government/publications/trading-goods-regulated-under-the-new-approach-if-theres-no-brexit-deal/trading-goods-regulated-under-the-new-approach-if-theres-no-brexit-deal