Common sense would say that it's unlikely that a speedo is not a legal requirement. It's not covered by the MOT seemingly but is a general design requirement for vehicles used in the UK:
Fitting Regulation 35 Road Vehicles (Construction and Use) Regulations 1986 states that every motor vehicle shall be fitted with a speedometer except:
a vehicle having a maximum speed not exceeding 25 m.p.h.,
a vehicle which, at all times, is unlawful to drive at more than 25 m.p.h.,
an agricultural motor vehicle driven at not more than 20 m.p.h.,
a motor cycle not exceeding 100cc first used before 1st April 1984,
an invalid carriage first used before 1st April 1984,
a works truck first used before 1st April 1984,
any vehicle first used before 1st October 1937,
a vehicle fitted with an approved tachograph which is required or not.
Vehicles first used on or after 1st April 1984 the speedometer should be capable of indicating the speed in miles per hour and kilometres per hour. Vehicles may instead comply with EC Regulation (Community Directive) 97/39 or ECE Reg 39.
These directives stipulate the markings, graduations of the speedometer and refer to 75/443/EEC which specifies the tolerances.
The indicated speed must never be less than the true speed (it must read exact or high) and between 40km/h and 120km/h the error must not exceed 10% + 2.5 m.p.h. high (true speed/10 + 4kph).
This means at a true speed of 25mph or 40km/h the speedometer may read 40/10+4 = 8km/h or 5mph high = 30mph indicated.
By using non standard wheels and tyres you could be had for an under reading speedo based on the above.