No it isn't a business expense. That's because the amount paid to the pension company is net of tax. The pension company claims the tax relief back from government. So if you pay in £80 the tax relief tops it up to £100 and that's the amount invested
It it was allowed as a business expense as well, you'd get two lots of tax relief.
Note that the situation is different when a company pays a contribution for it's employees (including directors)
You can claim personal pension relief if you are a high rate tax payer. You cannot claim if you normally pay tax at 20%. This would not be declared as an expense.
You would complete the tax relief section of the return. You would need to find out if your employer is giving you tax relief through your pay roll. You do not declare contributions taken from your pay before it was taxed.
Also, thanks to Anonymous for the comments above. This is correct but it’s also worthwhile adding that an expense is something which is required to be paid in order to carry out the job. A pension contribution does not fall into this and would never be regarded as an expense for an individual.