In law you were entitled to a written statement of your terms and conditions within a set period...not the ad hoc way your employer appears to have attempted.
The offering of a pay slip by an employer would usually be argued by a trade union anyway as constituting a contract. They should have given you a written statement of terms and conditions of your post - name, post title, place of employment, remuneration, frequency, type, holiday leave, company policies (including the handbook). Such document could usually have a clause re: right to vary conditions etc. Document should have been signed and dated by employer. A letter of appointment may/may not have spelled out such items????
On the 'constructive' dismissal - be careful that you can show that you tried, maybe severally, to address these issues with the employer. They are notoriously difficult cases to prove to any satisfaction.
Check with your local citizens centre. Make an appointment. Read their website.
Good luck too.