BIK, bonuses & commission ARE included in the calculation of normal pay.
Here's the legislation if you're interested:
16. (i) In the case of an employee who is paid wholly or partly by piece rates, bonuses or commissions (being piecerates, bonuses or commissions related directly to his output) and in the case of any other employee whose remuneration varies in relation to the amount of work done by him, his normal weekly remuneration shall be taken to be the amount as calculated in accordance with subparagraph (ii).
(ii) For the purposes of subparagraph (i) normal weekly remuneration shall be calculated by dividing the remuneration to be taken into account in accordance with subparagraph (iii) by the number of hours ascertained in accordance with subparagraph (vi) and multiplying the resulting hourly rate by the normal weekly working hours of the employee concerned at the date on which he was declared redundant.
(iii) The remuneration to be taken into account for the purposes of subparagraph (ii) shall be the total remuneration paid to the employee concerned for all the hours worked in the period of 26 weeks which ended 13 weeks before the date on which the employee was declared redundant, adjusted in respect of any variations in the rates of pay which became operative during the period of 13 weeks ending on the date on which the employee was declared redundant.